logging for DNS resolvers?

[davepacheco]

While debugging oxidecomputer/omicron#8466, I went looking for log entries for the DNS resolution but I didn't find any. I think it'd be worthwhile and not too chatty to have log entries when qorb attempts to make DNS queries, gets responses, changes the set of backends, etc. It definitely seems worthwhile to have an entry when the resolver gets shut down, especially via drop and not terminate(). These could definitively answer a lot of questions about potential DNS issues after-the-fact.