R1:9f35c7cc2675

Complete modularization of the GC daemon

Summary: This modularizes the rest of the GC submethods. Turned out there was nothing tricky.

Test Plan: Ran `bin/phd debug garbage` and got reasonable looking behavior and output.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7971