Fix: keep ust/kernel session items around for destroy notifier

Fix: keep ust/kernel session items around for destroy notifier

Split the destruction and release operations for ust and kernel
inner-sessions as they may be accessed by session destruction
notifiers registered against their parent ltt_session.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>