Introduce DifferentialChangesetQuery and remove loadHunks()

Summary: Ref T4045. Ref T5179. This removes all non-Query hunk loads.

Test Plan:

• Viewed revisions.
• Viewed standalone changesets.
• Viewed raw old/new files.
• Viewed vs diffs.
• Enabled inline comments in mail and sent some transactions with inlines.
• Called differential.getrawdiff.
• Grepped for loadHunks().

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4045, T5179

Differential Revision: https://secure.phabricator.com/D9289