title: Backbone.js: define timeout for Backbone.sync implementation
body: <p>I'm using backbone on a project of mine, integrated with communication to an external API. I want to use real-time updating of records. Since I don't have access to the main backend of this external application, and they don't provide neither websocket server nor long-polling endpoint, I am basically left with the option of doing regular polling with setInterval and a period of 50 seconds. It has been working quite well. My problem is the edge case. If for some reason the API request hangs, for more than 50 secs, let's say, I'll be triggering a new request right away. That means, 2 hanging requests now, which will add up eventually. Is there a way to set a timeout for the request? I know all requests lead to Backbone.sync, but I was checking the source code and I don't see any feasible way of setting the timeout for the XmlHttpRequest. Is there a way to do this cleanly and without overwriting behaviour? Or are there other solutions/workarounds?</p>