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/template/template.go

@@ -75,7 +75,7 @@ func (h Handler) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 
 		msg := new(dns.Msg)
 		msg.SetReply(r)
-		msg.Authoritative, msg.RecursionAvailable, msg.Compress = true, true, true
+		msg.Authoritative, msg.RecursionAvailable = true, true
 		msg.Rcode = template.rcode
 
 		for _, answer := range template.answer {
@@ -105,6 +105,7 @@ func (h Handler) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 		}
 
 		state.SizeAndDo(msg)
+		state.Scrub(msg)
 		w.WriteMsg(msg)
 		return template.rcode, nil
 	}