Route reply handler Maniphest writes through new transaction code

Summary: Ref T2217. Mail writes go through the new code now.

Test Plan: Shipped a bunch of mail in with `./bin/mail`, got reasonable edits as a result.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2217

Differential Revision: https://secure.phabricator.com/D7086