Allow transactions to require read locks during edits

Summary:
Let applicationtransaction editors request locks when applying certain transactions. See D5567.

We should probably do this for all not-guaranteed-safe operations, but let's put our toe in the water with Conpherence first. I believe the cost of these locks is very small and my cautious stance toward acquiring them isn't really warranted, but I've also never seen, e.g., a race on a title edit.

Test Plan: Added comments with and without locks, verified both pathways worked correctly.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D5582