Update PolicyBasedChaosMonkey to handle uncaught exceptions

Running ServerKillingChaosMonkey via RESTApiClusterManager for any duration of time slowly leaks region servers. I see failures on the RESTApi side go unreported on the ChaosMonkey side. It seems like RuntimeExceptions are being thrown and lost.

PolicyBasedChaosMonkey uses a primitive means of thread management anyway. Update to use a thread pool, thread groups, and an uncaughtExceptionHandler.