Enabled some functional tests now that end-to-end scenarios work and fixed bugs

Or: How I Learned to Stop Worrying and Love the test TODOs,

We had some functional tests that were not running waiting on Migrations support for:
* Candidate keys
* Sequences
* Default values

These are now enabled. Fixed two bugs while doing this:
* Double-dispatch was not being performed for default value SQL gen resulting in inccorect literal generation
* Not all places that needed to check for whether or not a property is a key were updated to support candidate keys.