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/cache.go

@@ -113,7 +113,6 @@ func (w *ResponseWriter) WriteMsg(res *dns.Msg) error {
 		cacheSize.WithLabelValues(Denial).Set(float64(w.ncache.Len()))
 	}
 
-	setMsgTTL(res, uint32(duration.Seconds()))
 	if w.prefetch {
 		return nil
 	}