Do Compress only when need in request.Scrub (#1760)

* Remove Compress by default

Set Compress = true in Scrub only when the message doesn not fit the
advertized buffer. Doing compression is expensive, so try to avoid it.

Master vs this branch
pkg: github.com/coredns/coredns/plugin/cache
BenchmarkCacheResponse-2   	   50000	     24774 ns/op

pkg: github.com/coredns/coredns/plugin/cache
BenchmarkCacheResponse-2   	  100000	     21960 ns/op

* and make it compile

plugin/etcd/stub_handler.go

@@ -32,8 +32,9 @@ func (s Stub) ServeDNS(ctx context.Context, w dns.ResponseWriter, req *dns.Msg)
 	if e != nil {
 		return dns.RcodeServerFailure, e
 	}
-	m.RecursionAvailable, m.Compress = true, true
+	m.RecursionAvailable = true
 	state.SizeAndDo(m)
+	m, _ = state.Scrub(m)
 	w.WriteMsg(m)
 	return dns.RcodeSuccess, nil
 }