Supporting compacted event types (#154)

[ePaul]

This is implementing #154.

• add tests
• 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.
• 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? → support compacted event types (#154) via spring beans #164
• after #142 Update flyway and spring boot #143 is merged, adjust all the imports in the tests (and possibly other code as well).

[ePaul]

For the snapshot triggering mechanism, we have the SnapshotEventGenerator interface, which an application needs to implement and provide as spring beans.

Here instead, the CompactionKeyExtractor (+ event-Type + Class) has to be passed to the registerCompactionKeyExtractor method on the EventLogWriter.

This can be seen as somewhat inconsistent. Should we try to do the same here?

• We get a wrapper class for these three things.
• The EventLogWriterImpl would get a list of these as constructor parameter.
• The Autoconfiguration collects beans of this type to inject them into the ELWImpl.

Then we can get rid of this register-method completely, and the interface of EventLogWriter would not change at all.

[ePaul]

This is just a glorified Function<X, String>.

[ePaul]

Unused variable.

Suggested change

	@Captor
	private ArgumentCaptor<EventLog> eventLogCapture;

[ePaul]

Replaced by #164.