fix: GREEN - Postponed the resetting of the sink

fix: GREEN - Postponed the resetting of the sink

* The completion of a sink of an instance of `ChatRoomData`, that belongs
  to a revoked partition is postponed until the following partition-
  assignment happens.
* Also, the sink is only completed and recreated, if the partition it
  belongs to, was not assigned to the instance during
  `onPartitionsAssigned()` again.
* That way, listeners have a little more time to receive the messages, that
  were send shortly prior to the deactivation, and are not disturbed
  unnecessarily, if the partition is reassigned to the same instance again.
* *TODO:* Listeners still have to be enabled, to start listening on the
  responsible instance where they left of on the old instance, if they
  were to slow to receive all messages, before the sind was completed.