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
exactly, those are from channels

Related thought, if `Persistent` messages were stored with `Processor` it does not make sense to be able to replay them into a `PersistentActor`. The semantics are different and not something we should support, even though it might work. The migration path for such things is to use latest 2.3.x (which has both `Processor` and `PersistentActor`) and let a `Processor` replay and forward to a `PersistentActor` that stores corresponding events (new persistenceId). Pretty much the migration tool we talked about (but lacking stream convenience and back-pressure).