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