Updating notification_id to native pointer size.

Prevents compile warning -Wint-to-void-pointer-cast
Fixing retried flag being incorrectly set. 

Previously the command would be added to the inflight
list then immediately checked if inflight.
Initialising is_waiting flag to false. 

is_waiting was uninitialised and starting as true.
Caused an early incorrect fire of sem_post.
Unlinking semaphores to prevent sem_open failure.

Need a better strategy for making sure the 
destructors are called.
Merge branch 'master' of https://github.com/ajwlucas/avdecc-lib