=per #3933 Correction of seq number logic for persistent channel

* Problem when using PersistentChannel from Processor
* When the seq numbers of the sending processor and the seq numbers
  of the PersistentChannel was out of sync the PersistentChannel
  did not de-duplicate confirmed deliveres that were resent by
  the processor.
* There is a hand-off in the RequestWriter that confirms the
  Processor seq number, and therefore the seq number of the
  RequestWriter must be used in the ConfirmablePersistent from
  the RequestReader
* More tests, covering this scenario
Merge pull request #2099 from akka/wip-3933-persistent-channel-patriknw

=per #3933 Correction of seq number logic for persistent channel