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