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