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