Fix: reference counting of consumer output

Fix: reference counting of consumer output

The UST app session has a reference on the consumer output object, but
it belongs to the UST session. Implement a refcounting scheme to ensure
it is not freed before all users are done using it.

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