As described in #371, all services should implement a "readiness" endpoint, which checks whether the service is ready to serve traffic, i.e. whether it can reach all of its dependencies.

• Add readiness endpoint for agent. It calls both build-index and tracker's readiness endpoints. They in turn call the origin's readiness endpoint. The endpoint succeeding provides a strong signal that an agent on a host is ready to provide images.

Stacked on top of #380