Flushes the flow and all its requests to the data_store.

def Close(self):
    """Flushes the flow and all its requests to the data_store."""
    self._CheckLeaseAndFlush()
    super(FlowBase, self).Close()
    # Writing the messages queued in the queue_manager of the runner always has
    # to be the last thing that happens or we will have a race condition.
    self.FlushMessages()