Write test that reproduces issue #24.  Also rewrite test to run faster and be simpler.  Minor change to ChangeTracker to support being inspected by test.

https://github.com/couchbase/couchbase-lite-android-core/issues/24
Fix for issue #24.  Unit test added in last commit now passes.

https://github.com/couchbase/couchbase-lite-android-core/issues/24