support for zone transfer for kubernetes (#1259)

* plugin/kubernetes: axfr

Hook up dynamic SOA serial by setting ResourceEventHandlerFuncs in
dnsController.

Add prototype of returns msg.Services via the Transfer function. Leave
expanding this to RRs out of scope for a bit.

* plugin/kubernetes: axfr

Respond to AXFR queries

* Fixing race condition

* Fixing instantiation of dnstest writer

* Updates from review

plugin/etcd/xfr.go

@@ -4,6 +4,8 @@ import (
 	"time"
 
 	"github.com/coredns/coredns/request"
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
 )
 
 // Serial implements the Transferer interface.
@@ -15,3 +17,8 @@ func (e *Etcd) Serial(state request.Request) uint32 {
 func (e *Etcd) MinTTL(state request.Request) uint32 {
 	return 30
 }
+
+// Transfer implements the Transferer interface.
+func (e *Etcd) Transfer(ctx context.Context, state request.Request) (int, error) {
+	return dns.RcodeServerFailure, nil
+}