Method executor

bolt/exec/tests/SpillerTest.cpp:998–1009

  }

  folly::IOThreadPoolExecutor* executor() {
    static std::mutex mutex;
    std::lock_guard<std::mutex> l(mutex);
    if (executorPoolSize_ == 0) {
      return nullptr;
    }
    if (executor_ == nullptr) {
      executor_ =
          std::make_unique<folly::IOThreadPoolExecutor>(executorPoolSize_);
    }
    return executor_.get();
  }

  void reset() {
    spiller_.reset();