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. 
fixed. created and linked up proper `onceDataChannelClosed` method, and no longer have a peerClosedChannel event (we can't actually distinguish in WebRtc when that happens vs when network goes down, etc.).