Multi-DB support

cache-machine did not respect multiple databases as intended by https://github.com/jbalogh/django-cache-machine/commit/b0f49d5ac8419cc7db1d38e74e64a400e5977625 and the tests partially hid this.  I was able to extend the tests to cover multiple databases (and removed the test-utils dependency) and was then able to fix the behavior.