docs: def FreezeTimestamp(self):
      """Freezes the timestamp used for resolve/delete database queries.
  
      Frozen timestamp is used to consistently limit the datastore resolve and
      delete queries by time range: from 0 to self.frozen_timestamp. This is
      done to avoid possible race conditions, like accidentally deleting
      notifications that were written by another process while we were
      processing requests.
      """
      self.prev_frozen_timestamps.append(self.frozen_timestamp)
      self.frozen_timestamp = rdfvalue.RDFDatetime.Now()