code: public KafkaClient setExecutorService(ExecutorService executorService) {
          if (this.executorService != null) {
              this.executorService.shutdown();
          }
          this.executorService = executorService;
          myOwnExecutorService = false;
          return this;
      }
docs: Sets an {@link ExecutorService} to be used for async task.
  
  @param executorService
  @return