middleware/cache: fix race (#757)

While adding a parallel performance benchmark I stumbled on a race
condition (another reason to add performance benchmarks!), so this
PR makes sure the msg is created in a race free manor and adds the
parallel benchmark.

middleware/cache/handler.go

@@ -29,6 +29,7 @@ func (c *Cache) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 	i, ttl := c.get(now, qname, qtype, do)
 	if i != nil && ttl > 0 {
 		resp := i.toMsg(r)
+
 		state.SizeAndDo(resp)
 		resp, _ = state.Scrub(resp)
 		w.WriteMsg(resp)