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.
WSS does not support this message. The supported way to remove a user is sending "DELETE" to https://apprtc-ws.webrtc.org:8089/{roomid}/{clientid}