trigger: use condition and action ref counting to ease internal objects management

trigger: use condition and action ref counting to ease internal objects management

Currently a trigger object has multiple relationship types with the action and
condition associated to it.

On the API client side, the trigger does not own the action and condition
associated to it. The user is responsible for managing the lifetime of the
associated action and condition object.

On the sessiond side, triggers created from `lttng_trigger_create_from_payload`
currently require that before calling lttng_trigger_destroy, the action and
condition be fetched and deleted using their respective destructor. This
operation cannot be done inside `lttng_trigger_destroy` since the exposed API is
clear that the trigger does no own the objects.

We can facilitate the lifetime/ownership management of the action and condition
objects using their respective reference counting mechanism.

On a trigger creation, the trigger get references on both object. On destroy,
the trigger put the references on the objects.

From an API client perspective, nothing changes. Even better, it prevents
premature freeing of these objects, since the trigger have references to these
objects.

On the sessiond side, we can now move the actual ownership of the action and
condition object to the trigger object. This is done in
`lttng_trigger_create_from_payload` forcing a put of the local references to the
object, effectively moving the ownership to the trigger object.

Note that the `lttng_trigger_get_{action, condition}` do not `get` a reference
to the object before returning it. This is done to comply with the API that was
introduced back in 2.11 which does expect a client to call lttng_{action,
condition}_destroy on the returned object.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I31d196d00c886fcf42c1b16c55e693949373568b