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.
Ah, I misunderstand the 'bye'. Now I see that it's sent to the other client. We should keep it then.
The DELETE to GCE server is still needed.