Discard VCLs with cold temperature (not cold state).

After vcl_cooldown, VCLs since Varnish at least 6.3 go to cold
temperature, but stay in the auto state.

Cold state happens when a VCL is set to cold via vcl.state, in which
case the temperature also goes cold. So an admin might hasten the
discard by setting the cold state.

pkg/varnish/monitor.go

@@ -154,7 +154,7 @@ func (vc *Controller) checkInst() bool {
 		return false
 	}
 	for _, vcl := range vcls {
-		if vcl.State == admin.ColdState {
+		if vcl.Temperature == admin.ColdTemp {
 			if err = adm.VCLDiscard(vcl.Name); err != nil {
 				vc.errorEvt(discardErr,
 					"Error discarding VCL %s: %v",