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 build index, which checks whether all its backends are reachable and the origin's readiness.
Add a client for build-index's readiness endpoint. The client will be used by agent -- agent's readiness endpoint will check build index's readiness, as build index is a dependency for agent.
Stacked on top of https://github.com/uber/kraken/pull/373
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.