netdir: New function to check consistency of a HasRelayIds
This function will be used to look up a relay by a set of LinkSpecs given from an incoming HsDesc or INTRODUCE2 message. It differs from other "lookup relay by IDs" functions in that it needs to be able to return "here's a relay", "couldn't found a relay", or "learned that this relay is impossible."
Closes #855 (closed): This is the only new API needed for ChanTarget validation, I think.