refactor(dialer): it should close idle connections (#457)

Like we did before for the resolver, a dialer should propagate the
request to close idle connections to underlying types.

See https://github.com/ooni/probe/issues/1591

internal/cmd/oohelperd/internal/websteps/generate.go

@@ -22,7 +22,7 @@ type Generator interface {
 
 // DefaultGenerator is the default Generator.
 type DefaultGenerator struct {
-	dialer     netxlite.Dialer
+	dialer     netxlite.DialerLegacy
 	quicDialer netxlite.QUICContextDialer
 	resolver   netxlite.ResolverLegacy
 	transport  http.RoundTripper