docs: def system_drop_column_family(self, column_family):
      """
      drops a column family. returns the new schema id.
  
      Parameters:
       - column_family
      """
      self._seqid += 1
      d = self._reqs[self._seqid] = defer.Deferred()
      self.send_system_drop_column_family(column_family)
      return d