The default dns.Response implementation of a dns.ResponseWriter will
panic if RemoteAddr() is called after the connection to the client has
been closed already. The current cache implementation doesn't create a
new request+responsewriter during an asynchronous prefetch, but
piggybacks on the request triggering the prefetch.

This change copies the RemoteAddr first, so that it's safe to use it
later during the actual prefetch request.

A better implementation would be to completely decouple the prefetch
request from the client triggering a request.