Add build index readiness endpoint and client

[Anton-Kalpakchiev]

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 Add origin readiness endpoint #373

[gkeesh7]

Some minor comments otherwise LGTM