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/ssh/ssh_graceful.go

@@ -24,5 +24,5 @@ func listen(server *ssh.Server) {
 
 // Unused informs our cleanup routine that we will not be using a ssh port
 func Unused() {
-	graceful.Manager.InformCleanup()
+	graceful.GetManager().InformCleanup()
 }