Add Stub resolving

SkyDNS can forward requests from one instance to another.
Add this base infrastructure for this feature to CoreDNS.

Add more tests as well.

middleware/etcd/etcd.go

@@ -17,7 +17,7 @@ import (
 type Etcd struct {
 	Next       middleware.Handler
 	Zones      []string
-	Proxy      proxy.Proxy
+	Proxy      proxy.Proxy // Proxy for looking up names during the resolution process
 	Client     etcdc.KeysAPI
 	Ctx        context.Context
 	Inflight   *singleflight.Group