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
well, they are actually put back into the proper sequence after async writing to the store, see AsyncWriteJournal.scala L31

That doesn't have to be reflected in the name of this trait, though. As far as I can see this type is only used in `WriteMessages`. It is part of the internal `JournalProtocol` so we are free to change.

`PersistentEnvelope`?