Supporting compacted event types (#154)

[ePaul]

This is implementing #154.

• [x] add tests
• [x] check whether/how this can be supported for snapshot sending
  • → this will work out of the box, if the data objects for the snapshots are the same type as the normal objects.
• [x] add/update documentation
• [ ] (maybe) bump version number (if this doesn't get into 21.0.0)
• [ ] Do we want a "key extractor as a bean" feature similarly to the snapshot generators? → #164
• [x] after #143 is merged, adjust all the imports in the tests (and possibly other code as well).

[ePaul]

Replaced by #164.