=per #15472 Flush instead of unstash when PersistentActor restarts

* When using the Processor batching buffer for persistAsync and defer
  the events were unstashed by Processor.aroundPreRestart and
  thereby were received as commands after restart
* Instead we must flush this buffer when using PersistentActor
Merge pull request #15473 from akka/wip-15472-unstashed-persistent-patriknw

=per #15472 Flush instead of unstash when PersistentActor restarts