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
The first question will be answered by the comment once we made sure that the one-by-one unstashing actually happens at the right times (which I’m not yet convinced of, see below in processingCommands).

Thanks for the reminder about `unstash()`, I only quickly looked at how it appears in Eclipse (i.e. for Java users) where it is public (yuck!) and asked myself whether everybody understands the conditions under which this method may be called.