Add a garbage collector daemon

Summary:
Phabricator generates a bunch of data that we don't need to keep around forever,
add a GC daemon to get rid of it with some basic configuration options.

This needs a couple more diffs to get some of the details but I think this is a
reasonable start.

I also fixed a couple of UI things related to this, e.g. the daemon logs page
going crazy when a daemon gets stuck in a loop and dumps tons of data to stdout.

Test Plan:

• Ran gc daemon in 'phd debug' mode and saw it delete stuff, then sleep once

it had cleaned everything up.

• Mucked around with TTLs and verified they work correctly.
• Viewed gc'd transcripts in the web interface and made sure they displayed

okay.

• Viewed daemon logs before/after garbage collection.
• Running some run-at / run-for tests now, I'll update if the daemon doesn't

shut off in ~10-15 minutes. :P

Reviewed By: tuomaspelkonen
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, tuomaspelkonen, epriestley
Differential Revision: 583