title: MongoDB ChangeStream
body: <p>I have serval questions for MongoDB ChangeStream(on version 3.6):</p>
  <ul>
  <li>a. When I start a MongoDB ChangeStream at Time T, MongoDB will returned changes on Which timing point?</li>
  <li>b. Will two ChangeStream subscription conflict for the same collection?</li>
  <li>c. Can I use resumeToken on ChangeStream A  to ChangStream B, in order to subscribe the same cursor of the Document changes?</li>
  </ul>