added stop-promise to socks-rtc-net

 * Added promise support for when socks-rtc stops (e.g. peer-connection breaks or is closed by remote peer).
Yeah, you're right. feels like typescript should supported read-only vars in its interfaces... but you're right, we should use a function for now. Right now this is being done for consistency with other libraries. I'll submit a pull request to change them all at once, but I'll do that separately.