Improve performance of two recent commit migrations

Summary:
Ref T13216. See PHI959. These two recent migrations can be expressed more efficiently:

• When updating commit audit statuses, the field isn't JSON encoded or anything so we can just issue several bulk UPDATEs.
• When inserting mail keys, we can batch them in groups of 100.

Test Plan: Used bin/storage upgrade -f --apply phabricator:... to reapply patches. Saw equivalent behavior and faster runtimes.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13216

Differential Revision: https://secure.phabricator.com/D19802