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
I have a feeling that this `var` can be eliminated, either by just removing the functionality or by checking `pendingStashingPersistInvocations > 0` in `doAroundReceive`.

On the first suggestion: when a write is currently pending, how much do we gain in latency by submitting the `persist` writes immediately instead of waiting until the acknowledgement comes in? The other thought is that sync and async persistence will rarely be mixed within the same Actor, in which case this flag actually does nothing (unless I am missing something).