ltt-sessiond teardown cleanup

ltt-sessiond now kills (SIGTERM) the kconsumerd before exiting, so the
LTTng module unload succeeds. Previously, the kconsumerd was still
holding a reference on ltt-relay.ko -- it was only being released after
ltt-sessiond closed its communication socket with kconsumerd.

Now, all the teardown actually wakes each thread, getting them to
return, and main() joins each of them before calling cleanup().
Previously, we had no guarantee that threads would not run concurrently
with cleanup() executed from the signal handler.

It would still be good to audit _each_ read call so we ensure that they
are performed on O_NONBLOCK fcntl fds (as we did in the kconsumerd), so
all the blocking is done in a poll() call, which can be woken up by the
sessiond termination file descriptor too (from the signal handler).
Failure to respect this could lead to races where sessiond blocks on a
file descriptor and never really honour the signal for a thread, thus
leading to blocking of the main() thread on a join.

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