This resolves #51.  This dramatically simplifies the PrioritySchedulerLimiter and reduces the code involved by constructing the returned future at submission time, and avoiding the whole "FutureListernableFuture" thing.
Removing these "FutureFuture" implementations.  They are complicated, and hard to use correctly.

In addition now that threadly no longer needs them, I think it would be advisable for others that need them to look towards other solutions.