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

integrations/migration-test/migration_test.go

@@ -6,6 +6,7 @@ package migrations
 
 import (
 	"compress/gzip"
+	"context"
 	"database/sql"
 	"fmt"
 	"io/ioutil"
@@ -220,7 +221,7 @@ func doMigrationTest(t *testing.T, version string) {
 	err := models.SetEngine()
 	assert.NoError(t, err)
 
-	err = models.NewEngine(wrappedMigrate)
+	err = models.NewEngine(context.Background(), wrappedMigrate)
 	assert.NoError(t, err)
 	currentEngine.Close()
 }