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/erratic/erratic.go

@@ -45,7 +45,6 @@ func (e *Erratic) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 
 	m := new(dns.Msg)
 	m.SetReply(r)
-	m.Compress = true
 	m.Authoritative = true
 	if trunc {
 		m.Truncated = true