docs: 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