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.
Would rather leave the change for not waiting for channelReady when we move to new server logic. Was just planning to have this replace Channel API as a signaling channel for now.