plugin/dnstap: some cleanup (#1172)

Some cleanup in proxy and dnstap:
* just use time pkg directly and side step the indirection for Epoch
* Use Set in SetQueryEpoch to be more Go like. (Looked like a reader)
* Don't maintain two sets of time, we already track start, so use that.
* Use time.Time and convert when needed
* dedent the toDnstap function and put in a separate file

plugin/dnstap/handler.go

@@ -79,7 +79,7 @@ func (h Dnstap) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 	newCtx := context.WithValue(ctx, DnstapSendOption, &sendOption)
 
 	rw := &taprw.ResponseWriter{ResponseWriter: w, Tapper: &h, Query: r, Send: &sendOption}
-	rw.QueryEpoch()
+	rw.SetQueryEpoch()
 
 	code, err := plugin.NextOrFailure(h.Name(), h.Next, tapContext{newCtx, h}, rw, r)
 	if err != nil {