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.
I think it makes more send now to replace /message with /bye, and the request body can empty.
Also I'd format the request like https://apprtc.appspot.com/bye/{roomid}/{clientid} to make it more REST-like, and more like the GCE API.