docs: public void addScannerServers(final int query_index, final int id, 
        final Set<String> servers) {
      Map<Integer, Set<String>> query_servers = scanner_servers.get(query_index);
      if (query_servers == null) {
        query_servers = new ConcurrentHashMap<Integer, Set<String>>(
            Const.SALT_WIDTH() > 0 ? Const.SALT_BUCKETS() : 1);
        scanner_servers.put(query_index, query_servers);
      }
      query_servers.put(id, servers);
    }