experimental prerelease 21.0.0-RC3b

This is an experimental release, containing one possible implementation of #154.

It is not clear yet whether this will become part of the final 21.0.0, of whether we go the other route (in 21.0.0-RC3a).

Change compared to 21.0.0-RC2:

• #164: support compacted event types (via spring beans)
  You now can submit events to a compacted event type, by adding a CompactionKeyExtractor spring bean.