Policy which decides how to process a received message or message batch. Example: pass the message or batch to
the thread pool (default impl), or pass only one (unicast and mulicast) message per sender to the thread pool at a
time and queue others from the same sender.
Process a message that was not received from the transport but from above (e.g. the channel or a protocol), and
needs to be looped back up because (1) the destination address is null (every multicast message is looped back)
or (2) the destination address is the sender's address (unicast message to self).
A message that is looped back can bypass cluster name matching.