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 Revision: https://secure.phabricator.com/D14601