Adjust keys for new Differential inline comment table

Summary:
Ref T2222. This sequences before D7139 and sorts out keys on the table. In particular:

• There was a fairly silly draft key modeled after Pholio; drop it.
• Add a revisionPHID key. This is queried mostly-transitionally on the revision view screen.
• Add a changesetID key. This is queried by a bunch of interfaces that want more surgical results than revisionPHID provides.
• Add an authorPHID, transactionPHID key. This is queried on the list interface to find pending drafts.
• Add a legacy key. This is queried by the feed publisher.

Test Plan: Used the query analyzer to hit all (I think?) of the pages, saw keyed queries.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D7140