Graceful: Xorm, RepoIndexer, Cron and Others (#9282)

* Change graceful to use a singleton obtained through GetManager instead of a global.
* Graceful: Make TestPullRequests shutdownable
* Graceful: Make the cron tasks graceful
* Graceful: AddTestPullRequest run in graceful ctx
* Graceful: SyncMirrors shutdown
* Graceful: SetDefaultContext for Xorm to be HammerContext
* Avoid starting graceful for migrate commands and checkout
* Graceful: DeliverHooks now can be shutdown
* Fix multiple syncing errors in modules/sync/UniqueQueue &  Make UniqueQueue closable
* Begin the process of making the repo indexer shutdown gracefully

modules/graceful/net_unix.go

@@ -101,7 +101,7 @@ func CloseProvidedListeners() error {
 // creates a new one using net.Listen.
 func GetListener(network, address string) (net.Listener, error) {
 	// Add a deferral to say that we've tried to grab a listener
-	defer Manager.InformCleanup()
+	defer GetManager().InformCleanup()
 	switch network {
 	case "tcp", "tcp4", "tcp6":
 		tcpAddr, err := net.ResolveTCPAddr(network, address)