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/engine/experiment/websteps/tcp.go

@@ -8,7 +8,7 @@ import (
 )
 
 type TCPConfig struct {
-	Dialer   netxlite.Dialer
+	Dialer   netxlite.DialerLegacy
 	Endpoint string
 	Resolver netxlite.ResolverLegacy
 }