fix testGetReplicator() issue

https://github.com/couchbase/couchbase-lite-android/issues/398
prevent overlapping saveCheckpoints

https://github.com/couchbase/couchbase-lite-android/issues/398
if user calls stop() on a replicator, it needs to stop the change tracker

was seeing tons of change tracker methods in logs when running tests

https://github.com/couchbase/couchbase-lite-android/issues/398