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