[fetch] Add option to reject the fetch promise automatically after a certain time elapsed (with no API for arbitrary aborts) (#179)

I'd like an option to have a **deadline** for the server to respond, so that if the response doesn't arrive within a specified number of seconds, the fetch promise is rejected and *internally* (i.e. without any sort of abort/cancellation API exposed to JS) the request is aborted by the browser on the TCP/IP level, so that bandwidth is not wasted in case the response does come later.

```js
fetch('/slow', {deadline: 5000})
  .then(called_if_response_is_quick, called_if_response_is_too_slow);
```

It'd be less wasteful equivalent of:

```js
const oldfetch = fetch;
fetch = function(input, opts) {
    return new Promise((resolve, reject) => {
        setTimeout(reject, opts.deadline);
        oldfetch(input, opts).then(resolve, reject);
    });
}
```

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/179