Lower the priority of event listeners

[priyadi]

Currently HandleMonitorStampListener has the same priority as DoctrineClearEntityManagerWorkerSubscriber

bin/console debug:event-dispatcher WorkerMessageHandledEvent

Registered Listeners for "Symfony\Component\Messenger\Event\WorkerMessageHandledEvent" Event
============================================================================================

 ------- -------------------------------------------------------------------------------------------------------- ---------- 
  Order   Callable                                                                                                 Priority  
 ------- -------------------------------------------------------------------------------------------------------- ---------- 
  #1      Zenstruck\Messenger\Monitor\EventListener\HandleMonitorStampListener::handleSuccess()                    0         
  #2      Symfony\Bridge\Doctrine\Messenger\DoctrineClearEntityManagerWorkerSubscriber::onWorkerMessageHandled()   0         
 ------- -------------------------------------------------------------------------------------------------------- ---------- 

In my installation above, HandleMonitorStampListener will be executed first.

The problem is that if there are unflushed entities, messenger-monitor-bundle will inadvertently flush them. On the other hand, without messenger-monitor-bundle, DoctrineClearEntityManagerWorkerSubscriber will clear these pending entities, and they won't be flushed. So we are getting different behaviors depending on whether messenger-monitor-bundle is installed or not.

I propose lowering the priority of HandleMonitorStampListener to something like -100, so that DoctrineClearEntityManagerWorkerSubscriber always runs earlier to clear the unflushed entities.

Also, ORMStorage should also call clear() after flush().