docs: private FutureRequestExecutionService buildRequestExecutorService(final CloseableHttpClient httpClient) {
          if (this.executorService == null) {
              this.executorService = new ThreadPoolExecutor(this.threadsNumber, this.threadsNumber, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<>(this.queueSize));
          }
          return new FutureRequestExecutionService(httpClient, this.executorService);
      }