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
@rkuhn I see what you mean now. It was not exercised by the new test because we used `defer`, which will invoke the handler immediately when there are no pendingInvocations, i.e. resequenceableEventBatch was still empty.
Changing to `persistAsync` the test fails exactly as you say. Thanks for an awesome observation.