Improve performance of "phabricator:20210215.changeset.02.phid-populate.php"

Summary: Ref T13613. Improve the performance of this migration by using a temporary table and an "UPDATE x JOIN y ..." pattern.

Test Plan:

• Ran on secure, got exit after a few seconds since the migration is idempotent and changesets already had PHIDs.
• Ran on secure with the continue; commented out, got valid new PHIDs in 53s (from 153s).
• Tried a larger page size (16K), didn't see any improvement.
• From "--trace", client PHID generation seems to be the limiting factor.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13613

Differential Revision: https://secure.phabricator.com/D21570