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 just realised that this should fire when the peer closes the channel *and* when it closes locally. It's the equivalent of `DataChannel.onceClosed`, right?