Fix disappearing ARender annotations

Steps to reproduce:

• Navigate to the annotations tab of a document on which there are many different types of annotations.
• Navigate away from the browser window for several minutes.
• Go back to the browser window.
  => Annotations have disappeared from the annotations browser.
  => "There is no annotation in this document !" [sic] appears in the annotations browser.
  => Browser console is flooded with repeatd error messages:

  arendergwt-0.js:2251 Wed Jun 05 17:41:16 GMT+200 2019 com.arondor.viewer.client.WebSocketManager
  SEVERE: Socket has been closed! 
  

See attached screenshot.