Allow sharing sessions across domains

If you try to share the session store across other application mounted for other domain (ie: sharing the session between domains under the same node instance) 

You can't beacause this in session.js

    // generates the new session
    store.generate = function(req){
      req.sessionID = utils.uid(24);
      req.session = new Session(req);
      req.session.cookie = new Cookie(cookie);
    };

If you ejecute session() to get the middleware in different applications, the last will override the generate method so something like this wouldn't work in the receiving application. 

    express.session({ store: sharedSessionStore });
    ...
    // later on during the request, we clear the cookie 
    // so we force to generate a new session
    res.clearCookie("connect.sid", { domain: "domain.com" });
    // with the passed session id 
    req.sessionID = sessionToHijackID;

If this provocates security issue, would be nice to have an alternative to deal with the shared sessions.