Implement the basics of a bootstrap-status API.

The purpose of this API is to tell the user how far along Arti is in getting bootstrapped, and if it's stuck, what it's stuck on.

This API doesn't yet expose any useful information: by the time it's observable to a client, it's always "100% bootstrapped." But I'm putting it in a MR now so that we can review the basic idea, and to avoid conflicts with later work on tickets like #293 (closed) and #278 (closed).

This is part of #96 (closed).