Sent messages are deleted individually after a crash

* If all messages with a sequence-number lower than or equal than the
  sequence number of a message already send are deleted, this can only
  be done for the lowest sequence-number seen accross all partitions
  without violating the at-least-once semantics.
* Deleting seen messages individually by specifying their id (aka
  sequence-number) explicitly in the delete-command allows to delete
  _all_ messages from the outbox, that have been seen on the topic,
  hence, achieving exactly-once-semantics even in the case of an
  unclean shutdown (aka crash) of the OutboxProducer