actions: Make lttng_action reference countable

lttng_action objects will be shared with the action executor subsystem
which executes them asynchronously. This introduces an ambiguous
ownership since triggers could be unregistered while an action is
executing (or pending execution).

Also ease the object ownership management of the group action sub
actions. This allow clients to add multiple time the same action to an
action group without any problem.

The currently user-visible 'destroy' method simply calls the 'put'
method which preserves the expected behaviour for current users (both
internal and public) of the API.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I2d016a9b80e418d40c72db8e155c44e96852b33f