Fix msg-bittorrent's problem of incompletion

In the previous version, a peer wasn't able to handle more than
more message per second but could receive much more. As a consequence,
the application gets stuck because of queuing issue.

The problem comes from teh main loops (leech_loop() and seed_loop()))
each call of MSG_task_irecv() was followed by a call of
MSG_process_sleep(1) because the MSG_comm_test() always fails the
first time.

A call of MSG_mailbox_set_async() at the begining fixes this problem.

: in the main loops (leech_loop)