R1:56be70056111

Improve code structure of PHID fields in EditEngine

Summary: Ref T9132. I had some hacks in place for dealing with Edge/Subscribers stuff. Clean that up so it's structured a little better.

Test Plan:
- Edited subscribers and projects.
- Verified things still show up in Conduit.
- Made concurrent edits (added a project in one window, removed it in another window, got a clean result with a correct merge of the two edits).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9132

Differential…