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 would pass 'false' to make the xhr synchronous to increase the chance of successful cleanup.
Then I'd call disconnectFromWSS() before disconnectFromGAE()