[WIP] Refactor django eager load listeners.

[jmcarp]

Currently, eager load listeners only check the associations loaded and ignore the specific objects. That means we can get false positives on empty queries, and touching an eager-loaded attribute can resolve the wrong eager load. This PR refactors eager load checks and expands test cases to handle situations that fail now.

• [x] Refactor django eager load
• [x] Expand django eager load tests
• [x] Refactor sqlalchemy eager load
• [x] Expand sqlalchemy eager load tests
• [x] Document the confusing bits