Use a gevent pool for actions syncback instead of Redis-plus-workers.

Summary:
This is much simpler, and we were probably going to change this eventually
anyways. The immediate reason for doing it right now is that the rq module
reconfigures the root logger, which is really annoying when you are trying to
actually configure it yourself.

Also, start the SyncbackService as a simple greenlet instead of a ZeroRPC
service (which it isn't really).

Test Plan: Unit tests updated, end-to-end syncback behavior manually verified.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D229