dnsdist: Stop reading on incoming DoH sockets if we are done

dnsdist: Stop reading on incoming DoH sockets if we are done

If the underlying library doesn't want to read, we have nothing
to send at the moment, but we are still waiting for responses,
there is no need to keep polling the socket.

Signed-off-by: Remi Gacogne <remi.gacogne@powerdns.com>
(cherry picked from commit e0a5a6dc4ab9557442ddbba56d03f6e4fb132bf2)