Did an initial implementation of scheduleAtFixedRate.  This does not currently have any unit testing (and thus is not ready to be merged into the main development branches)
Catch this exception from recurring tasks if we are shutting down.
Merge branch 'unstable_staging' into PriorityScheduledExecutorServiceWrapper-scheduleAtFixedRate
Starting to increase functionality in the TestRunnable to track usage in parallel (which currently can only be done from the TestRunnable, although that too is being improved).
More improvements to the TestRunnable for handling when the run is finished, as well as detecting if it ran concurrently.
Implemented unit tests for this new functionality.  I also emphisized that using this functionality should be with caution (but better than just throwing an UnsupportedOperationException)
Reduced some code duplication now that TestRunnable is capable of accepting a delay on construction.
Further code duplication in unit tests reductions.  In addition solved a unit test failure due to always using the native sleep in TestRunnable (which I want to keep since it should represent computational work).  So needed to bring back the SleepThread inner class to TestablePrioritySchedulerTest.