Improve status for cases where mock server has no queued response.

https://github.com/couchbase/couchbase-lite-java-core/issues/271
Add assertions regarding changes counts in tests.  (all pass)

https://github.com/couchbase/couchbase-lite-java-core/issues/271
Rename test, add minor assertions regarding all docs being pulled.

https://github.com/couchbase/couchbase-lite-java-core/issues/271