make churn and its samples work with freedom.js v0.6

Pretty much the same as the other sample apps.

Depends on:
https://github.com/uProxy/uproxy-lib/pull/90

One thing I noticed about churn is that it didn't like RFC 6544-style "TCP candidates". So I've made `extractEndpointFromCandidateLine` a little more tolerant.
Do you feel that way about all the Promises on WebRtc.PeerConnection and WebRtc.DataChannel?  I don't see what's so bad about them.