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/server_hooks.go

@@ -14,15 +14,15 @@ import (
 // awaitShutdown waits for the shutdown signal from the Manager
 func (srv *Server) awaitShutdown() {
 	select {
-	case <-Manager.IsShutdown():
+	case <-GetManager().IsShutdown():
 		// Shutdown
 		srv.doShutdown()
-	case <-Manager.IsHammer():
+	case <-GetManager().IsHammer():
 		// Hammer
 		srv.doShutdown()
 		srv.doHammer()
 	}
-	<-Manager.IsHammer()
+	<-GetManager().IsHammer()
 	srv.doHammer()
 }