R1:6899fbcf2966

Add DifferentialHunkQuery to start hiding hunk storage details

Summary:
Ref T4045. We have a lot of direct queries against the hunk table right now. These are messy, not really policy-aware, and limit our options on T4045.

This query is unusual (it requires changesets, and does not accept IDs). This keeps us from having to load changeset -> diff -> revision in order to do policy checks. We could also fix this with smarter policy checks and caching, but I'd rather not open that can of worms for now. This…