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

cmd/migrate.go

@@ -5,6 +5,8 @@
 package cmd
 
 import (
+	"context"
+
 	"code.gitea.io/gitea/models"
 	"code.gitea.io/gitea/models/migrations"
 	"code.gitea.io/gitea/modules/log"
@@ -32,7 +34,7 @@ func runMigrate(ctx *cli.Context) error {
 	log.Trace("Log path: %s", setting.LogRootPath)
 	setting.InitDBConfig()
 
-	if err := models.NewEngine(migrations.Migrate); err != nil {
+	if err := models.NewEngine(context.Background(), migrations.Migrate); err != nil {
 		log.Fatal("Failed to initialize ORM engine: %v", err)
 		return err
 	}