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).
This is true, but not an issue introduced in this.

FWIW, most (if not all) ntp implementations just prevent the clock moving forward, unless it is grossly out of sync, in which case yes it will go backwards.

I have considered using System.nanoTime() as it is arbitrary and will never go backwards.  But right now that is not implemented.  I will file a github issue to cover this though.