debug action "PREPARE:WAIT" could cause deadlock and query cannot be cancelled

Status ExecNode::ExecDebugAction() {
...
  if (debug_action_ == TDebugAction::WAIT) {
    while (!state->is_cancelled()) {
      sleep(1);
    }
    return Status::CANCELLED;
  }
...
}

void PlanFragmentExecutor::Cancel() {
  VLOG_QUERY << "Cancelling plan fragment...";
  lock_guard<mutex> l(prepare_lock_);
  is_cancelled_ = true;
  if (!is_prepared_) {
    VLOG_QUERY << "Cancel() called before Prepare()";
    return;
  }
  DCHECK(runtime_state_ != NULL);
  VLOG_QUERY << "Cancel(): instance_id=" << runtime_state_->fragment_instance_id();
  runtime_state_->set_is_cancelled(true);
  runtime_state_->stream_mgr()->Cancel(runtime_state_->fragment_instance_id());
}