Kill the long polling transport's ping loop in connection.stop

- Before, if the connection was stopped while in the ping loop (which
  is normal when the server goes offline) and then manually started before
  a ping succeeded, a zombie poll loop would result.

#2840
Abort active pings in connection.stop

- This helps prevent zombie poll loops

#2840
JS unit test to verify that active pings are canceled on stop.

#2840