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/kubernetes/kubernetes.go

@@ -47,6 +47,7 @@ type Kubernetes struct {
 	primaryZoneIndex   int
 	interfaceAddrsFunc func() net.IP
 	autoPathSearch     []string // Local search path from /etc/resolv.conf. Needed for autopath.
+	TransferTo         []string
 }
 
 // New returns a initialized Kubernetes. It default interfaceAddrFunc to return 127.0.0.1. All other