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 don't think the TODO description is right. We should always send the DELETE no matter if the websocket has registered. Since disconnecting websocket without sending DELETE will make the server start a timer and remove the client after the 5s timeout, which is a waste.