Add a `(dst, type, src)` key to Differential's edge table

Summary:
Ref T1279. This came to me in a dream.

The existing `differential_relationship` table has an `(objectPHID, type)` column, which theoretically is useful for queries like "revisions with X as a reviewer". In practice, I'm not sure it gets used much, but I can get it to show up in at least some query plans.

Add a similar index to the `edge` table. This sequences //before// D7227, which actually migrates the data.

Test Plan:
  - Ran `bin/storage upgrade`.
  - EXPLAIN'd a bunch of queries against different versions of the schema, this seemed helpful overall.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1279

Differential Revision: https://secure.phabricator.com/D7232

Conflicts:
	src/infrastructure/storage/patch/PhabricatorBuiltinPatchList.php
Migrate all Differential reviewer data to edges

Summary:
Ref T1279. @champo did a lot of this work already; we've been doing double writes for a long time.

Add "double reads" (reading the edge table as both the "relationship" table and as the "reviewer status" table), and migrate all the data.

I'm not bothering to try to recover old reviewer status (e.g., we could infer from transactions who accepted old revisions) because it wold be very complicated and doesn't seem too valuable.

Test Plan:
  - Without doing the migration, used Differential. Verified that reads and writes worked. Most of the data was there anyway since we've been double-writing.
  - Performed the migration. Verified that everything was still unchanged.
  - Dropped the edge table, verified all reviweer data vanished.
  - Migrated again, verified the reviewer stuff was restored.
  - Did various cc/reviewer/subscriber queries, got consistent results.

Reviewers: btrahan

Reviewed By: btrahan

CC: champo, aran

Maniphest Tasks: T1279

Differential Revision: https://secure.phabricator.com/D7227