Add etcd middleware

This middleware acts in the same way as SkyDNS. We might add options
to allow it to be behave different, but for now it will suffice.

A Corefile like:
.:1053 {
    etcd miek.nl
    proxy . 8.8.8.8:53
}
will perform lookup in etcd and proxy everything not miek.nl to Google
for further resolution.

The internal etcd forwarding *also* uses the proxy infrastructure,
meaning you get health check and such for (almost) free

middleware/state.go

@@ -83,6 +83,30 @@ func (s State) Family() int {
 	return 2
 }
 
+// Do returns if the request has the DO (DNSSEC OK) bit set.
+func (s State) Do() bool {
+	if o := s.Req.IsEdns0(); o != nil {
+		return o.Do()
+	}
+	return false
+}
+
+// UDPSize returns if UDP buffer size advertised in the requests OPT record.
+// Or when the request was over TCP, we return the maximum allowed size of 64K.
+func (s State) Size() int {
+	if s.Proto() == "tcp" {
+		return dns.MaxMsgSize
+	}
+	if o := s.Req.IsEdns0(); o != nil {
+		s := o.UDPSize()
+		if s < dns.MinMsgSize {
+			s = dns.MinMsgSize
+		}
+		return int(s)
+	}
+	return dns.MinMsgSize
+}
+
 // Type returns the type of the question as a string.
 func (s State) Type() string {
 	return dns.Type(s.Req.Question[0].Qtype).String()