Improve GC performance for Herald Transcripts

Summary: This has to table scan a ginormous table right now, give it a fighting
chance with a more usable key.
Test Plan:
  - Launched GC daemon, no errors.
  - Used test console to create a new transcript.
  - Viewed some old transcripts.
  - Ran EXPLAIN on the SELECT and verified it was utilizing the garbageCollected
key.

Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, epriestley, jungejason
Differential Revision: 735

resources/sql/patches/069.heraldxscript.sql

@@ -0,0 +1,9 @@
+ALTER TABLE phabricator_herald.herald_transcript
+  ADD garbageCollected BOOL NOT NULL DEFAULT 0;
+
+UPDATE phabricator_herald.herald_transcript
+  SET garbageCollected = 1
+  WHERE objectTranscript = "";
+
+ALTER TABLE phabricator_herald.herald_transcript
+  ADD KEY (garbageCollected, time);