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 maybeStart should not be blocked on channelReady, since it can start sending messages before channelReady.
I'd remove this call and modify maybeStart to not check channelReady.