Add unit test to detect issue #164. Also enhance test to use public API.

https://github.com/couchbase/couchbase-lite-android/issues/164
add testDocumentCache() in attempt to repro issue #164, but in this case it did not detect any issues.
temporarily remove document cache from Database

The minimal set of changes required to remove the document cache from
the Database class. It's ugly, but it should be temporary (until the
cache is invalidated everywhere it needs to be).

See couchbase/couchbase-lite-android#164.