Bug 1025872 — Setup websockets for call setup information

The API has been defined by @adamroach in this document: https://wiki.mozilla.org/Loop/Architecture/MVP#WebSockets_Connection_for_Call_Progress
There is two solutions about this problem:
 - either we have one redis connection and a on(message) per websocket in a call setup (since we have removeListener)
 - either we have a redis connection per client with a on(message) per connection.

I think this solution is better.