def _doCleanup(self):
        """
        Perform any periodic database cleanup tasks.

        @returns: Deferred
        """
        # pass on this if we're not configured yet
        if not self.configured_url:
            return

        d = self.changes.pruneChanges(self.master.config.changeHorizon)
        d.addErrback(log.err, 'while pruning changes')
        return d
Perform any periodic database cleanup tasks.

        @returns: Deferred