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

models/branches.go

@@ -5,6 +5,7 @@
 package models
 
 import (
+	"context"
 	"fmt"
 	"time"
 
@@ -525,7 +526,8 @@ func (deletedBranch *DeletedBranch) LoadUser() {
 }
 
 // RemoveOldDeletedBranches removes old deleted branches
-func RemoveOldDeletedBranches() {
+func RemoveOldDeletedBranches(ctx context.Context) {
+	// Nothing to do for shutdown or terminate
 	log.Trace("Doing: DeletedBranchesCleanup")
 
 	deleteBefore := time.Now().Add(-setting.Cron.DeletedBranchesCleanup.OlderThan)