fix: The sink is recreated, if it gets canceled

fix: The sink is recreated, if it gets canceled

- The sink is automatically cancelled, if the last consumer leaves.
- Turning auto-cancel of, is not an option, because this buffers new
  messages, until the buffer overflows, after which new messages are
  droped. Hence, if a new subscriber arrives, it would see some old
  messages and then, after a gap, current messages.
- Because of that, the sink is now automatically recreated, if the
  last subscriber leaves and triggers the automatically cancelation
  of the sink.
- The sink can be recreated without conflicts, because all methods,
  that may access the sink are synchronized.