Channel and journal plugin API enhancements

- more efficient bookkeeping during delivery and redelivery
- different confirm messages for transient and persistent channels
- journal plugin API now fully batch-oriented
  * batch-write of confirmations
  * added batch deletions

Todo

- actual batching of delivery confirmations
- actual batching of deletions
- reliable channel enhancements as discussed
Prepare journal API for paged replay

- separate replay from sequence number recovery
- add an atMost parameter to asyncReplayMessages
- add an replayAtMost parameter to recoveryStarted state