R1:4ef918e213ed

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…