Mostly modularize the Differential "update" transaction

Summary: Ref T13099. Move most of the "Update" logic to modular transactions

Test Plan: Created and updated revisions. Flushed the task queue. Grepped for TYPE_UPDATE. Reviewed update transactions in the timeline and feed.

Maniphest Tasks: T13099

Differential Revision: https://secure.phabricator.com/D19175