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/handler.go

@@ -53,6 +53,8 @@ func (k Kubernetes) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.M
 			break
 		}
 		fallthrough
+	case dns.TypeAXFR, dns.TypeIXFR:
+		k.Transfer(ctx, state)
 	default:
 		// Do a fake A lookup, so we can distinguish between NODATA and NXDOMAIN
 		_, err = plugin.A(&k, zone, state, nil, plugin.Options{})
@@ -76,6 +78,7 @@ func (k Kubernetes) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.M
 	m.Extra = append(m.Extra, extra...)
 
 	m = dnsutil.Dedup(m)
+
 	state.SizeAndDo(m)
 	m, _ = state.Scrub(m)
 	w.WriteMsg(m)