title: How to update only the non-existing documents and delete the old documents which doesnot exist now using mongoimport
body: <p>I am using <code>mongoimport</code> in order to import a bunch of JSONs and I am looking for a way 
  to</p>
  
  <ol>
  <li>only to import records that don't exist </li>
  <li>keep the matching records </li>
  <li>delete the old records (which are not in the newly inserted jsons)</li>
  </ol>