Revisit model loading

[tomchop]

All objects that are loaded from the DB are laoded into a "rich" TBB object. This is particularly slow because of how STIX objects do checks all over the place.

It would be faster to have JSON returned from the database and have the api opt-in on returning a rich object.

Another alternative would be to have callers opt-out from rich objects (e.g. for the list API endpoint).