Initial websocket notifications experiment

    Initial websocket notifications experiment
    
    Add EnginioBackendConnection which implements a minimally needed
    WebSocket protocol to be able to read incoming messages from a
    specified backend.
    
    Add a test to set up a backend with the additional option:
    
    settings: {
        websocket: true  # this enables websocket notifications
    }
    
    This currently only works with the staging server.