Implement channel fd monitoring thread for UST

This new thread uses a new hash table containing channels and monitors
their wait_fd. On hang up, the channel streams are closed but *not*
deleted so the teardown process of channel and streams are untouched.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>