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
Hmm, strange, need to think more. But after a `persistAsync` or `defer` your condition of `resequenceableEventBatch.size <= pendingStashingPersistInvocations` does not hold. Could it be that persistence is too fast in your test? You can make it slower by also adding a Latch to the Cmd to ensure that `"b"` is enqueued before persistence returns.