Make fixtures using foreign keys easy to use.

Foreign keys are currently very painful to use in the test suite. You have to manage the order of your fixtures very carefully, and sometimes even that is not enough.

This set of changes adds a way to disable/enable keys from the connection. No specific tests were added as the basic behavior of the fixture manager did not change. Instead, I added foreign keys to a fixture to help ensure this continues working.
Thanks for adding those. I was lazy at the beginning :)