Similar to JGRP-2417.
Required by Infinispan which uses Netty's pooled buffers (ByteBuf). This needs to be given a callback to release a ByteBuf back to the pool. The callback should be invoked when JGroups is done with a message M, ie. when M is removed from the retransmission tables (e.g. in NAKACK2 or UNICAST3).

Provide new (default: no-op) methods in Message:

• incr(): increments the reference count
• decr(): decrements the reference count
• refCount(): gets the reference count

Method incr() is called when a message is sent, decr() when (1) NAKACK or UNICAST release the message (removal from the retransmission table) and (2) when the send returns.

When the reference count goes from 1 to 0, the resource associated with the message (e.g. a pooled bytebuf) can be released.