Should kill running Spark Jobs when a query is cancelled.

  public synchronized void shutdown() {
    LOG.debug("Shutting down query " + ctx.getCmd());
    shutdown = true;
    for (TaskRunner runner : running) {
      if (runner.isRunning()) {
        Task<?> task = runner.getTask();
        LOG.warn("Shutting down task : " + task);
        try {
          task.shutdown();
        } catch (Exception e) {
          console.printError("Exception on shutting down task " + task.getId() + ": " + e);
        }
        Thread thread = runner.getRunner();
        if (thread != null) {
          thread.interrupt();
        }
      }
    }
    running.clear();
  }