User supplied Queues and Executors

Desire to allow user to supply their own queue to Bukkit/Sponge implementations.

Major Issue:
Right now, a factory is expected to be 1 to 1 relation to its queue.
if factory shuts down, the queue shuts down.

But what if 2 factories share the same queue?

How do we process shutdown of a factory and block until its tasks are flushed from the async queue but allow the other factories to continue?