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/git/git_test.go

@@ -5,6 +5,7 @@
 package git
 
 import (
+	"context"
 	"fmt"
 	"os"
 	"testing"
@@ -16,7 +17,7 @@ func fatalTestError(fmtStr string, args ...interface{}) {
 }
 
 func TestMain(m *testing.M) {
-	if err := Init(); err != nil {
+	if err := Init(context.Background()); err != nil {
 		fatalTestError("Init failed: %v", err)
 	}