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).
Should make the internal promise (`private onceStopped_`), and then expose a `public onceStopped()` method. This prevents any chance of the user of `uproxy-networking` has no risk of accidentally overwriting the promise itself, making it useless. (e.g Potentially doing something like `socksToRtc.onceStopped = someFunction`) It would be more clear if it must always be `socksToRtc.onceStopped().then( ... )