Transactions - make TYPE_COMMENT implementation optional
Summary: Ref T6403. Conpherence keeps track of comments for message counts so we needed some special attention there. Otherwise, straight-forward.
Test Plan: left a comment on a diff with inline comments. sent messages in conpherence successfully. verified unread count incremented correctly for sent messages for users.
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T6403
Differential Revision: https://secure.phabricator.com/D12932