Every so often, ask the Aphict server how things are going

Summary: Ref T12573. This sends a "ping" to the server, and a "pong" back to the client, every 15 seconds. This tricks ELBs into thinking we're doing something useful and productive.

Test Plan: Ran bin/aphlict debug, loaded Phabricator, saw ping/pong in logs.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12573

Differential Revision: https://secure.phabricator.com/D17717