Add check for route handlers hanging requests

[aseemk]

It's possible to write an Express route that forgets to return a response in a certain case. E.g.:

app.get('/foo', function (req, res, _) {
    if (...) {
        res.render('foo');
    }
});

If this case is hit, the request will just hang, waiting for a response.

Express-Streamline could probably add a convenience check for this case. E.g. when a route handler "returns" (in Streamline parlance), if a response hasn't been sent (I'm assuming the response object will have some props we can inspect for this?), throw an error as it's probably a bug.

There's a chance it's not a bug, if the handler is doing something event-based and non-Streamline, but it's unlikely if you're using Express-Streamline. Still, we could make this an optional feature if so.