+per #3776 Live read models

- introduced Replicator that replicates the persistent message stream of a corresponding processor
- common code shared by Replicator and Processor factored out to Recovery and Snapshotter traits

Features

- pull based message replication via journal
- replication based on incremental replays
- replication preserves message ordering
- replicators can have their own snapshots (independent of their replicated processor)
- replication done automatically based on a configurable interval
- replication can also be triggered by user

Todo

- Java API
- User docs