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/backend.go

@@ -3,6 +3,7 @@ package plugin
 import (
 	"github.com/coredns/coredns/plugin/etcd/msg"
 	"github.com/coredns/coredns/request"
+	"golang.org/x/net/context"
 
 	"github.com/miekg/dns"
 )
@@ -37,6 +38,10 @@ type Transferer interface {
 
 	// MinTTL returns the minimum TTL to be used in the SOA record.
 	MinTTL(state request.Request) uint32
+
+	// Transfer handles a zone transfer it writes to the client just
+	// like any other handler.
+	Transfer(ctx context.Context, state request.Request) (int, error)
 }
 
 // Options are extra options that can be specified for a lookup.