Takes a list of datastreams and a datetime, run undo save on all of them,
        and returns a list of the datastreams where the undo succeeded.

        :param datastreams: list of datastream ids (should be in self.dscache)
        :param logMessage: optional log message

def _undo_save(self, datastreams, logMessage=None):
        """Takes a list of datastreams and a datetime, run undo save on all of them,
        and returns a list of the datastreams where the undo succeeded.

        :param datastreams: list of datastream ids (should be in self.dscache)
        :param logMessage: optional log message
        """
        return [ds for ds in datastreams if self.dscache[ds].undo_last_save(logMessage)]