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 object is very low level and relatively unusual, and this small deviation from convention seems like the cleanest cut to make to keep this from snowballing.

Test Plan: Used Herald dry runs to verify that the affected rules still output the same data.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4045

Differential Revision: https://secure.phabricator.com/D8765