per #15423 Remove deprecated features from akka-persistence

* remove channels
* remove View
* remove Processor
* collapse the complicated internal state management
  that was spread out between Processor, Eventsourced and Recovery
* remove Recovery trait, this caused some duplication between Eventsourced
  and PersistentView, but but the enhanced PersistentView will not be based
  on recovery infrastructure, and therefore PersistentView code will be replaced anyway
* remove PersistentBatch
* remove LoopMessage
* remove deleteMessages of individual messages
* remove Persistent, PersistentRepr and PersistentImpl are kept
* remove processorId
* update doc sample code
> when will the internalStash be unstashed?

isn't that answered by the comment?

it's also unstashed in aroundPreRestart

> what about unstash()

I guess it is not overridden because it is not public api, but it I guess it wouldn't hurt to do it

> we are lacking a few tests in this area

I agree, I'll create a ticket