rework testChangeTrackerLongPoll() so that it makes sense.  remove testChangeTrackerContinuous since neither the test or the underlying code seemed to be working.
Get rid of MockHttpClientNeverResponds and use CustomizableMockHttpClient
improve testChangeTracker() to use countdown latch instead of sleep loop
Use public api for creating replicators
Add test failingTestChangeTrackerContinuous that exposes issue when using change tracker in continuous mode.  Disable continuous mode completely and don't use it in any other tests.