Swap Maniphest edit writes to new transactions

Summary: Ref T2217. This is essentially the last writer, should be able to start deleting code now.

Test Plan: Used "Edit Task" to make a bunch of task edits.

Reviewers: btrahan, garoevans

Reviewed By: garoevans

CC: aran

Maniphest Tasks: T2217

Differential Revision: https://secure.phabricator.com/D7090