More flexible StubResolver queries

[jeromegn]

For our use case, we're forwarding some requests to an unbound server running on our hosts.

The StubResolver doesn't quite work for us because:

• There's no way to send it a Message as the query, only a impl Question, therefore the response's id won't match our request's id.
• We can't directly modify the returned Message (from Answer::into_message) because the underlying Octets are Bytes which aren't mutable.

We presently work around this by writing the answer's bytes to a BytesMut, then using that as a Message's backing Octets and then modifying the header to set the original request's id.

Would it be desirable to add a function to make a DNS request via a full Message instead of just impl Question?

If not, then maybe a query_mut function to receive an Answer that can be converted to a Message<BytesMut>.

[partim]

The original intention for the stub resolver was to be a driver for the more complex logic implemented by the lookups. Allowing you to do your own raw query was a bit of a side effect. Since that seems to be one of the main uses now, it might be time to reconsider.

We have a plan to separate out the transport implementations into their own modules. This might be a good time to rethink the whole concept.