action executor: execute action only if the associated trigger is registered

action executor: execute action only if the associated trigger is registered

It was decided that a trigger removal should have immediate effect and
not result in a waiting on action queued to be executed.

There was two major way to do this:

1) On unregistrer trigger, lock the action executor queue and remove
all work-item related to the trigger ebing unregistered.

2) Mark the trigger as "unregistered" and validate its state when a work
item is ready for execution. If the trigger is marked as "unregistered",
simply consume and skip the work item.

To isolate the un-registration and the execution of a work item
depending on that trigger, we use the trigger lock to have exclusivity
during the execution for that trigger.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I9f42a323e0ec0dc51339f39f07536ec8060dc02a