Added scheduleAtFixedRate to SimpleSchedulerInterface

I found a way to implement this with minimal to no impact to the normal execution case.  By having this implemented in PriorityScheduler and SingleThreadScheduler, it makes their ExecutorServiceWrapper much simpler.

I doubt there are scheduler implementations for our interfaces that are outside of our project.  So I don't consider this a non-backwards compatible change.  It is designed to be an expansion of our feature set, as well as hopefully simplifying _some_ code (I guess while making others more complex).