Qname router

[Philip-NLnetLabs]

Proof of concept for query routing. The example query-routing shows how to route to different upstreams based on the qname. The client subnet option is copied from the request to upstream and in the reply back downstream again to show option processing.

This PR adds the following to unstable-server-transport:

• The trait SingleService which is a simplified service trait for requests that should generate a single response.
• The trait ComposeReply and an implementation of the trait (ReplyMessage) to assist in capturing EDNS(0) options that should be included in a response message.
• Adapters to implement Service for SingleService and to implement SingleService for SendRequest.
• Conversion of a Request to a RequestMessage.
• A sample query router, called QnameRouter, that routes requests based on the QNAME field in the request.

[bal-e]

This is a really cool feature! It took me some time to understand this part of the codebase, but looking at it now, everything seems to be quite straightforward. My primary concern is with the number of types that users need to interact with -- we should look for a way to simplify the interface.

[tertsdiepraam]

Some small suggestions on top of Arya's.