AppRTC: Use WebSockets instead of Channel API.

With this change no messages are stored on GAE. Instead, they are stored on Collider.
GAE still keeps track of which users are connected to a given room.
Note that loopback will no longer work.
Please add a comment to explain how loopback works, i.e. creating a new websocket client to mimic a fake user, who echos the received messages (except replacing 'offer' with 'answer') back to the sender.