Rewrite queue (#24505)

# ⚠️ Breaking

Many deprecated queue config options are removed (actually, they should
have been removed in 1.18/1.19).

If you see the fatal message when starting Gitea: "Please update your
app.ini to remove deprecated config options", please follow the error
messages to remove these options from your app.ini.

Example:

```
2023/05/06 19:39:22 [E] Removed queue option: `[indexer].ISSUE_INDEXER_QUEUE_TYPE`. Use new options in `[queue.issue_indexer]`
2023/05/06 19:39:22 [E] Removed queue option: `[indexer].UPDATE_BUFFER_LEN`. Use new options in `[queue.issue_indexer]`
2023/05/06 19:39:22 [F] Please update your app.ini to remove deprecated config options
```

Many options in `[queue]` are are dropped, including:
`WRAP_IF_NECESSARY`, `MAX_ATTEMPTS`, `TIMEOUT`, `WORKERS`,
`BLOCK_TIMEOUT`, `BOOST_TIMEOUT`, `BOOST_WORKERS`, they can be removed
from app.ini.

# The problem

The old queue package has some legacy problems:

* complexity: I doubt few people could tell how it works.
* maintainability: Too many channels and mutex/cond are mixed together,
too many different structs/interfaces depends each other.
* stability: due to the complexity & maintainability, sometimes there
are strange bugs and difficult to debug, and some code doesn't have test
(indeed some code is difficult to test because a lot of things are mixed
together).
* general applicability: although it is called "queue", its behavior is
not a well-known queue.
* scalability: it doesn't seem easy to make it work with a cluster
without breaking its behaviors.

It came from some very old code to "avoid breaking", however, its
technical debt is too heavy now. It's a good time to introduce a better
"queue" package.

# The new queue package

It keeps using old config and concept as much as possible.

* It only contains two major kinds of concepts:
    * The "base queue": channel, levelqueue, redis
* They have the same abstraction, the same interface, and they are
tested by the same testing code.
* The "WokerPoolQueue", it uses the "base queue" to provide "worker
pool" function, calls the "handler" to process the data in the base
queue.
* The new code doesn't do "PushBack"
* Think about a queue with many workers, the "PushBack" can't guarantee
the order for re-queued unhandled items, so in new code it just does
"normal push"
* The new code doesn't do "pause/resume"
* The "pause/resume" was designed to handle some handler's failure: eg:
document indexer (elasticsearch) is down
* If a queue is paused for long time, either the producers blocks or the
new items are dropped.
* The new code doesn't do such "pause/resume" trick, it's not a common
queue's behavior and it doesn't help much.
* If there are unhandled items, the "push" function just blocks for a
few seconds and then re-queue them and retry.
* The new code doesn't do "worker booster"
* Gitea's queue's handlers are light functions, the cost is only the
go-routine, so it doesn't make sense to "boost" them.
* The new code only use "max worker number" to limit the concurrent
workers.
* The new "Push" never blocks forever
* Instead of creating more and more blocking goroutines, return an error
is more friendly to the server and to the end user.

There are more details in code comments: eg: the "Flush" problem, the
strange "code.index" hanging problem, the "immediate" queue problem.

Almost ready for review.

TODO:

* [x] add some necessary comments during review
* [x] add some more tests if necessary
* [x] update documents and config options
* [x] test max worker / active worker
* [x] re-run the CI tasks to see whether any test is flaky
* [x] improve the `handleOldLengthConfiguration` to provide more
friendly messages
* [x] fine tune default config values (eg: length?)

## Code coverage:

![image](https://user-images.githubusercontent.com/2114189/236620635-55576955-f95d-4810-b12f-879026a3afdf.png)

custom/conf/app.example.ini

@@ -926,12 +926,6 @@ ROUTER = console
 ;; Global limit of repositories per user, applied at creation time. -1 means no limit
 ;MAX_CREATION_LIMIT = -1
 ;;
-;; Mirror sync queue length, increase if mirror syncing starts hanging (DEPRECATED: please use [queue.mirror] LENGTH instead)
-;MIRROR_QUEUE_LENGTH = 1000
-;;
-;; Patch test queue length, increase if pull request patch testing starts hanging (DEPRECATED: please use [queue.pr_patch_checker] LENGTH instead)
-;PULL_REQUEST_QUEUE_LENGTH = 1000
-;;
 ;; Preferred Licenses to place at the top of the List
 ;; The name here must match the filename in options/license or custom/options/license
 ;PREFERRED_LICENSES = Apache License 2.0,MIT License
@@ -1376,22 +1370,6 @@ ROUTER = console
 ;; Set to -1 to disable timeout.
 ;STARTUP_TIMEOUT = 30s
 ;;
-;; Issue indexer queue, currently support: channel, levelqueue or redis, default is levelqueue (deprecated - use [queue.issue_indexer])
-;ISSUE_INDEXER_QUEUE_TYPE = levelqueue; **DEPRECATED** use settings in `[queue.issue_indexer]`.
-;;
-;; When ISSUE_INDEXER_QUEUE_TYPE is levelqueue, this will be the path where the queue will be saved.
-;; This can be overridden by `ISSUE_INDEXER_QUEUE_CONN_STR`.
-;; default is queues/common
-;ISSUE_INDEXER_QUEUE_DIR = queues/common; **DEPRECATED** use settings in `[queue.issue_indexer]`. Relative paths will be made absolute against `%(APP_DATA_PATH)s`.
-;;
-;; When `ISSUE_INDEXER_QUEUE_TYPE` is `redis`, this will store the redis connection string.
-;; When `ISSUE_INDEXER_QUEUE_TYPE` is `levelqueue`, this is a directory or additional options of
-;; the form `leveldb://path/to/db?option=value&....`, and overrides `ISSUE_INDEXER_QUEUE_DIR`.
-;ISSUE_INDEXER_QUEUE_CONN_STR = "addrs=127.0.0.1:6379 db=0"; **DEPRECATED** use settings in `[queue.issue_indexer]`.
-;;
-;; Batch queue number, default is 20
-;ISSUE_INDEXER_QUEUE_BATCH_NUMBER = 20; **DEPRECATED** use settings in `[queue.issue_indexer]`.
-
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Repository Indexer settings
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
@@ -1418,8 +1396,6 @@ ROUTER = console
 ;; A comma separated list of glob patterns to exclude from the index; ; default is empty
 ;REPO_INDEXER_EXCLUDE =
 ;;
-;;
-;UPDATE_BUFFER_LEN = 20; **DEPRECATED** use settings in `[queue.issue_indexer]`.
 ;MAX_FILE_SIZE = 1048576
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
@@ -1441,7 +1417,7 @@ ROUTER = console
 ;DATADIR = queues/ ; Relative paths will be made absolute against `%(APP_DATA_PATH)s`.
 ;;
 ;; Default queue length before a channel queue will block
-;LENGTH = 20
+;LENGTH = 100
 ;;
 ;; Batch size to send for batched queues
 ;BATCH_LENGTH = 20
@@ -1449,7 +1425,7 @@ ROUTER = console
 ;; Connection string for redis queues this will store the redis connection string.
 ;; When `TYPE` is `persistable-channel`, this provides a directory for the underlying leveldb
 ;; or additional options of the form `leveldb://path/to/db?option=value&....`, and will override `DATADIR`.
-;CONN_STR = "addrs=127.0.0.1:6379 db=0"
+;CONN_STR = "redis://127.0.0.1:6379/0"
 ;;
 ;; Provides the suffix of the default redis/disk queue name - specific queues can be overridden within in their [queue.name] sections.
 ;QUEUE_NAME = "_queue"
@@ -1457,29 +1433,8 @@ ROUTER = console
 ;; Provides the suffix of the default redis/disk unique queue set name - specific queues can be overridden within in their [queue.name] sections.
 ;SET_NAME = "_unique"
 ;;
-;; If the queue cannot be created at startup - level queues may need a timeout at startup - wrap the queue:
-;WRAP_IF_NECESSARY = true
-;;
-;; Attempt to create the wrapped queue at max
-;MAX_ATTEMPTS = 10
-;;
-;; Timeout queue creation
-;TIMEOUT = 15m30s
-;;
-;; Create a pool with this many workers
-;WORKERS = 0
-;;
 ;; Dynamically scale the worker pool to at this many workers
 ;MAX_WORKERS = 10
-;;
-;; Add boost workers when the queue blocks for BLOCK_TIMEOUT
-;BLOCK_TIMEOUT = 1s
-;;
-;; Remove the boost workers after BOOST_TIMEOUT
-;BOOST_TIMEOUT = 5m
-;;
-;; During a boost add BOOST_WORKERS
-;BOOST_WORKERS = 1
 
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