Update middleware API

Middleware API error subscriptions now get the request object:

rxws.use()
    .subscribe(({req, res, reply, retry, next}) => {
        res.requestTime = Date.now();
        next();
    });

rxws.use()
    .subscribe(({req, res, reply, retry, next}) => {
        next();
    }, ({req, err}) => {
        // Do something with the error and the request.
    });