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
The difference should be visible with the following message sequence (immediately sending a reply upon reception of each message):

  * (1): `persist`
  * (2 contains a latch): await the latch, then `defer`
  * (3): nothing
  * wait for receipt of replies 1, 2
  * (4): nothing
  * open the latch
  * wait for the other two replies

I’m pretty certain that with your code the echoes will come in the order 1, 2, 4, 3 while with my change it will be 1, 2, 3, 4.