Fix: Assert on metadata generation failure

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Clean shutdown of lttng-live connection on SIGINT/SIGTERM

Implement the lttng_live_recv and lttng_live_send wrappers which
are non-blocking and monitor a "should_quit" pipe. This allows us
to tear down the lttng-live connection cleanly.