R1:40e2a1c800ff

Write new hunks to the modern hunk store

Summary: Ref T4045. Ref T5179. Send all new writes into the modern store.

Test Plan:
- Created a diff.
- Verified it went to the modern store.
- Destroyed a revision, verified hunks were destroyed.
- Also unit tests.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4045, T5179

Differential Revision: https://secure.phabricator.com/D9293