title: create a custom sequential global dispatch queue
body: <p>In many places in my app I use the next code to perform background tasks and notify the main thread:</p>
  
  <pre><code>dispatch_queue_t backgroundQueue = dispatch_queue_create("dispatch_queue_#1", 0);
      dispatch_async(backgroundQueue, ^{
  
     dispatch_async(dispatch_get_main_queue(), ^{
  
  
          });
      });
  </code></pre>
  
  <p>Is it possible to create a backgroundQueue in one place (where does the best way?) and use it later?
  I know about the system global queue, but ordering is important for me. </p>