[SPARK-1685] Cancel retryTimer on restart of Worker or AppClient

See https://issues.apache.org/jira/browse/SPARK-1685 for a more complete description, but in essence: If the Worker or AppClient actor restarts before successfully registering with Master, multiple retryTimers will be running, which will lead to less than the full number of registration retries being attempted before the new actor is forced to give up.