=per #15937 Allow PersistentActor to be used as a stackable modification

 * PersistentActor correctly calls `super.around*` and allows to be mixed into e.g. ActorSubscriber
 * Tests have been added on PersistentActor and Processor to verify the stackable behavior of `around*` and `pre*` methods
 * Delegation in Processor has been simplified
Merge pull request #16126 from mwegrz/mwegrz-allow-persistentactor-to-be-used-as-a-stackable-modification

=per #15937 Allow PersistentActor to be used as a stackable modification