code: @Nullable
    private MasterWorkerInfo findUnregisteredWorker(long workerId) {
      for (IndexedSet<MasterWorkerInfo> workers: Arrays.asList(mTempWorkers, mLostWorkers)) {
        MasterWorkerInfo worker = workers.getFirstByField(ID_INDEX, workerId);
        if (worker != null) {
          return worker;
        }
      }
      return null;
    }
docs: Find a worker which is considered lost or just gets its id.
  @param workerId the id used to find a worker
  @return a {@link MasterWorkerInfo} which is presented in master but not registered,
  or null if not worker is found.