Use a separate thread to handle incoming messages.

Incoming messages were taken into account very late,
because of the following scheme:

	  ...
	* do something (compute, load-balance, ...)
	  ...
	* receive some message
	* create a new asynchronous receive communication

	Here, we cannot have received any new message, even
	if the sender was ready a long time ago, since the
	transfer only started when we created the new communication.
	Thus, we start a new iteration.

The solution is to implement a separate thread to handle incoming
messages asynchronously.  Those messages are put in a queue to be
consumed later.