sparts.tasks.queue: New PriorityQueueTask, UniqueQueueTask
sparts.tasks.queue: New PriorityQueueTask, UniqueQueueTask

Initial versions.  UniqueQueue has some race conditions wrt reliable
deferred/future processing that need to be addressed in a better way.

djipko/sparts,fmoo/sparts,fmoo/sparts,djipko/sparts,bboozzoo/sparts,pshuff/sparts,facebook/sparts,pshuff/sparts,facebook/sparts,bboozzoo/sparts