Automated progress tracking for stream-based event processing

[krasserm]

Write source read progress to event processing target logs. This requires an extension to ReplicationProtocol.ReplicationWrite that should contain the read progresses from n sources. Also, a batch-oriented writeReplicationProgresses should be added to EventLogSPI.

For reading progress values, provide a ProcessingProgressSource that can be created from a source log id and a target log and that emits a single progress value (see also Source.fromFuture)

See also http://rbmhtechnology.github.io/eventuate/adapters/stream.html#event-processing-progress-tracking