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.go

@@ -106,7 +106,8 @@ func SetExecutablePath(path string) error {
 }
 
 // Init initializes git module
-func Init() error {
+func Init(ctx context.Context) error {
+	DefaultContext = ctx
 	// Git requires setting user.name and user.email in order to commit changes.
 	for configKey, defaultValue := range map[string]string{"user.name": "Gitea", "user.email": "gitea@fake.local"} {
 		if stdout, stderr, err := process.GetManager().Exec("git.Init(get setting)", GitExecutable, "config", "--get", configKey); err != nil || strings.TrimSpace(stdout) == "" {