title: MongoWriteBatch:execute throws MongoCursorTimeoutException
body: <p>I'm performing bulk upserts to MongoDB using <code>MongoUpdateBatch</code></p>
  
  <pre class="lang-php prettyprint-override"><code>$batch = new \MongoUpdateBatch($collection, ['ordered' =&gt; false, 'wtimeout' =&gt; 500000]);
  </code></pre>
  
  <p>and occasionally get the following error:</p>
  
  <pre class="lang-none prettyprint-override"><code>Read timed out after reading 0 bytes, waited for 30.000000 seconds
  
  Trace:
  #8 MongoCursorTimeoutException in /var/app/current/src/Model/Campaign.php:286
  #7 MongoWriteBatch:execute in /var/app/current/src/Model/Campaign.php:286
  #6 ...
  </code></pre>
  
  <p>For what reason can <code>MongoWriteBatch:execute()</code> throw a <code>MongoCursorTimeoutException</code> and how can it be avoided?</p>