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 `PersistentBatch` was never sent to the journal, and never stored. It was a wrapper for `Persistent` messages that user could send to `Processor` as a way defined that these messages must be stored together. Internally the `Processor` unwrapped those and sent to the journal in `WriteMessages(Seq[Persistent])`.

I think the reason for being protobuffed was that `PersistentBatch` could be sent over remoting.