Close udp connection on stop

This MR is built on top of !43 (merged) which needs to be reviewed and merged first. !44 (59f234ee) is the relevant commit for this MR.

It fixes an "Address already in use" error, when trying to re-run the obfsvpn client integrated in the android app. Since the c.openvpnConn UDP connection was never explicitly closed, the port was still bound after the client was stopped.