Make the mock PUT checkpoint responder a bit smarter

https://github.com/couchbase/couchbase-lite-java-core/issues/231
Add ability to delay MockCheckpointPut to make behavior more realistic

https://github.com/couchbase/couchbase-lite-java-core/issues/231
Enhance mockSinglePull to detect failures related to issue.

https://github.com/couchbase/couchbase-lite-java-core/issues/231
Remove unneeded sleep from test

https://github.com/couchbase/couchbase-lite-java-core/issues/231
Re-enable assertions for tests

https://github.com/couchbase/couchbase-lite-java-core/issues/231