Resolve a peer's capabilities by live query #77

Some peers forbid retention — projectionTtl: 0 means do not cache, ask me each time. For those there is nothing stored, so a discover has to reach the peer at request time and answer from what comes back. A caller must not be able to tell which path answered it, and a peer being slow or unreachable has to fail in a way the caller can act on rather than looking like an empty result.

Doc: Discovery ("Discovery may use a live query or a permitted external projection"), Share (zero projectionTtl).

Parent: #48