Scaledown plus draining messages from one broker to another remaining broker

The message migration feature of the operator is one that is relatively unique to the broker operator and is something that is depended on to ensure message availability during statefutset scaledown.

The mechanism used, that of a scaledown controller that watches PVC's and launches an independent drainer pod, has been in use since early in the broker operator's lifetime. Since then a number of issues, as linked, have been raised that should be evaluated and actioned if required to improve upon the status quo.