Add lttng_notification_channel_has_pending_notification()

Add lttng_notification_channel_has_pending_notification()

This new API allows notification channel users to check for
pending notifications without necessarily blocking until
a new notification is ready. Moreoever, the pending notification
is not consumed by this new API.

lttng_notification_channel_get_next_notification() must still
be called to consume the new notification.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>