R1:96f9b0917e21

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…