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. 
ooo, as I thought about this I realized it's not what we want. We can't distinguish between various close events. Instead I added a onceDataChannelClosed method to give a promise back.