MM-27116: Drain push notification channel before closing (#15066)

* MM-27116: Drain push notification channel before closing

Without this, pending push notifications will not be sent when shutting down
or restarting the server.

* Fix race

* Re-arrange server shutdown

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/server.go

@@ -641,7 +641,6 @@ func (s *Server) Shutdown() error {
 	defer sentry.Flush(2 * time.Second)
 
 	s.HubStop()
-	s.StopPushNotificationsHubWorkers()
 	s.ShutDownPlugins()
 	s.RemoveLicenseListener(s.licenseListenerId)
 	s.RemoveClusterLeaderChangedListener(s.clusterLeaderListenerId)
@@ -659,6 +658,8 @@ func (s *Server) Shutdown() error {
 
 	s.StopHTTPServer()
 	s.stopLocalModeServer()
+	// Push notification hub needs to be shutdown after HTTP server
+	s.StopPushNotificationsHubWorkers()
 
 	s.WaitForGoroutines()