docs: def describe_schema_versions(self, ):
      """
      for each schema version present in the cluster, returns a list of nodes at that version.
      hosts that do not respond will be under the key DatabaseDescriptor.INITIAL_VERSION.
      the cluster is all on the same version if the size of the map is 1.
      """
      self._seqid += 1
      d = self._reqs[self._seqid] = defer.Deferred()
      self.send_describe_schema_versions()
      return d