No way to signal no change with ChangeEventHandler

[johanandren]

From forum question: https://discuss.lightbend.com/t/changeeventhandler-no-op/10620/3

The updateHandler has signature (State, State, Command) => ChangeEvent so there is no way to say a command had no effect on the state.

[johanandren]

Alternatively, if we want to require that a ChangeEvent is always returned, we should document that it isn't called for commands that doesn't change the state and show how to do a catch-ignore for those to satisfy the Scala (and Java) compiler completeness checker.

[patriknw]

Adding documentation makes sense.

The event sequence number must be in sync with the durable state change revision, i.e. one event per change. Therefore I think it is best to not make the ChangeEvent optional (even though we could store that hidden ignore payload).

Calling the change handler also for non-changes doesn't make sense, because we can't store additional events when there is no revision change. Again, sync of sequence numbers.