_scheduler: Slightly changing the Job / Queue API contracts

For the Job:

   o The action_cb, complete_cb and max_retries are now passed
     into the constructor, and the Job.spawn() API has no arguments.

   o The complete_cb() signature has changed to take a success
     boolean instead of a returncode integer, and also now receives
     the result directly.

For the Queue:

   o Adapted to new Job API contract changes

   o Changed the Queue.done() method to now take a success boolean
     instead of a returncode integer

Updated Queue implementations to handle the success boolean instead
of the returncode integer.