plugin/{kubernetes/etcd}: dynamic SOA (#1188)

Add a dynamic SOA record (at least the serial and minttl). This create
another interface that should be implemented by the backends.

For now default to returning epoch in as a uint32 (no change from
before).

Lower the minTTL returned to 30s (from 60s)

plugin/etcd/xfr.go

@@ -0,0 +1,17 @@
+package etcd
+
+import (
+	"time"
+
+	"github.com/coredns/coredns/request"
+)
+
+// Serial implements the Transferer interface.
+func (e *Etcd) Serial(state request.Request) uint32 {
+	return uint32(time.Now().Unix())
+}
+
+// MinTTL implements the Transferer interface.
+func (e *Etcd) MinTTL(state request.Request) uint32 {
+	return 30
+}