title: NEVPNManager's onDemandEnabled flag
body: <p>When <code>NEVPNProtocol</code> property is updated by the server such that the username &amp; password isn't valid anymore, is there a way we can handle this? </p>
  
  <p>Lets say if we turn on <code>onDemandEnabled</code> flag for NEVPNManager, and when the server invalidates the credential (since the user has been blacklisted), is there a way we can toggle this programmatically? From the end user's experience perspective, the user would wait for a long time to fail webpage loading. Background fetch using a timer might not work if the app is killed.</p>
  
  <p>Other than using push notifications, is there a better way to solve this issue?</p>