code: def _doCleanup(self):
          
          # 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
docs: Perform any periodic database cleanup tasks.
  
          @returns: Deferred