Modified the snowflake client to use pion/webrtc as the webrtc library.
This involved a few small changes to match function signatures as well
as several larger ones:
- OnNegotiationNeeded is no longer supported, so CreateOffer and
SetLocalDescription have been moved to a go routine called after the
other peer connection callbacks are set
- We need our own deserialize/serialize functions
- We need to use a SettingEngine in order to access the
OnICEGatheringStateChange callback

Rather than having it happen as a side effect of flag parsing.

Having it happen as a side effect of flag parsing caused log messages
related to handling the option (e.g. "IceServerList:") to always go to
stderr, despite the presence of a -log option. This is because the
requested log file had not been opened yet.

 - stabilize rest of WebRTCPeer
 - use uid for the datachannel label
 - store a consistent WebRTC config in the dialer

log traffic bytes only once every few seconds, along with OnMessage & datachannel.Send counts, to prevent flooded logs