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/proxy/dns.go

@@ -63,7 +63,6 @@ func (d *dnsEx) Exchange(ctx context.Context, addr string, state request.Request
 	if err != nil {
 		return nil, err
 	}
-	reply.Compress = true
 	reply.Id = state.Req.Id
 	// When using force_tcp the upstream can send a message that is too big for
 	// the udp buffer, hence we need to truncate the message to at least make it