Modularize the Garbage Collector

Summary:
The GC is a big block of hard-coded application GCs right now. Among other things, this means third parties can't tap into the infrastructure.

Modularize it into `GarbageCollector` classes. This implements only one to prove the new stuff works; I'll followup with the rest in the next diff or few depending on how much mess I run into.

Test Plan: Used `bin/phd debug garbage` to run the collector in debug mode, observed reasonable output and behavior.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7970