title: Cancel GCD block working in thread
body: <p>I have the following block which performs a request in the background.<br>
  How may I cancel this request before it has completed?</p>
  
  <pre><code>    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
      dispatch_async(queue, ^{
          NSData *thumbnailData = [NSURLConnection sendSynchronousRequest:request];
          ...
      });
  </code></pre>