The OutboxProducer restores the sequence-number from the wirtten topic

* The OutboxProducer consumes the written messages on start-up, extracts the
  send sequence-numbers and recalculates the seen watermarks and the maximal
  sequence-number, for at-least-once semantics.
* This implementation will most probably resend some messages after a crash,
  but it will never skip any unsend messages.
* Messages may be resend despite the recalculated watermarks, because only
  messages with a sequence number lower than the lowest watermark can be
  discarded safely from the outbox, if message-loss is not acceptable.
* Deactivated the integration-test for the loading of the context, because
  it cannot work without an available Kafka-Cluster.