title: Rxjs subscription queue
body: <p>I have a firebase subscription in my angular app which fires multiple times.
  How can ich achieve that the tasks are processed as a queue so that I can run each task synchronously once?</p>
  
  <pre><code>this.tasks.subscribe(async tasks =&gt; {
     for (const x of tasks) 
        await dolongtask(x); // has to be sync
        await removetask(x);
     });
  </code></pre>
  
  <p>The problem is that the subribe event fires when the longtask is still processing.</p>