title: convert list of completable futeres to one completable future of list
body: <p>I have a list of <code>CompletableFuture</code> instances.</p>
  
  <pre><code>List&lt;CompletableFuture&lt;String&gt;&gt; listOfFutures;
  </code></pre>
  
  <p>How is it to convert them to one future like this:</p>
  
  <pre><code>CompletableFuture&lt;List&lt;String&gt;&gt; futureOfList = convert(listOfFutures);
  </code></pre>