tweaked/idealized uproxypeerconnection interface

Tweaked the uproxypeerconnection interface so my work on socks-rtc compiles and updated samples to match.

TESTED: grunt and ran `src/coreproviders/samples/freedomchat-chromeapp/` manually, messages still flow, no console errors. 
I think this has to an event because multiple data channels may close over the lifetime of the peerconnection but promises can fulfill only once. How about `dataChannelClosed`?