(stable) Write edge transactions in a more compact way

Summary: Depends on D18946. Ref T13051. Begins writing edge transactions as just a list of changed PHIDs.

Test Plan: Added, edited, and removed projects. Reviewed transaction record and database. Saw no user-facing changes but a far more compact database representation.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13051

Differential Revision: https://secure.phabricator.com/D18947