Feature update and bug fixes

- Paged replay prototype using high/low watermark
  * incl API docs + tests

- Actor ref resolution in channels now preserves message order
  * fixes #3814
  * speedup delivery of new messages (no initial resolution)

- Message deletion enhancements
  * more efficient range deletions

- PersistentRepr.update now public (for non akka.persistence journals)

- Parameter for replaying logically deleted messages
  * needed for a journal compatibility kit (to be sometimes later)

Still missing:

- Persistent channel recovery optimizations (savepoints)
- Persistent channel user docs (are obsolete at the moment)
- Batching of confirmations and deletions