Author: Bela Ban
Version: $id$

A typical protocol in JGroups can be assured that only up() and down() methods can be called concurrently,
however up() and up() methods, and down() and down() methods cannot be called concurrently. This is due
to JGroups using an up and down queue, and 1 thread per queue continually dequeues messages and invokes up() or down().
However, with the introduction of down_thread=false and up_thread=false, we don't use queues anymore. If, in this case,
we have multiple threads calling Channel.send() concurrently, then the above guarantee cannot be provided anymore.
As a result, down() in any given protocol *can* now be called concurrently, by different threads, and so each protocol
has to be made thread safe (reentrant).

The focus here has to be on the down() side; multiple threads can call send() simultaneously.
However, the up() methods are probably not much of a problem because they are called by a single
thread (the receiver thread). We *cannot* use a thread pool for receiving messages, as this would
violate the ordering previously established.