message: [tf.data service] Only create journal writer when fault_tolerant_mode is enabled.
diff: @@ -71,23 +71,21 @@ Status CreateWorkerStub(const std::string& address, const std::string& protocol,
  DataServiceDispatcherImpl::DataServiceDispatcherImpl(
  const experimental::DispatcherConfig& config)
  : config_(config) {
  - if (!config_.work_dir().empty()) {
  - journal_writer_ = absl::make_unique<FileJournalWriter>(
  - Env::Default(), JournalDir(config_.work_dir()));
  - }
  }
  Status DataServiceDispatcherImpl::Start() {
  + mutex_lock l(mu_);
  if (!config_.fault_tolerant_mode()) {
  LOG(INFO) << "Running with fault_tolerant_mode=False. The dispatcher will "
  "not be able to recover its state on restart.";
  return Status::OK();
  }
  - mutex_lock l(mu_);
  if (config_.work_dir().empty()) {
  return errors::InvalidArgument(
  "fault_tolerant_mode is True, but no work_dir is configured.");
  }
  + journal_writer_ = absl::make_unique<FileJournalWriter>(
  + Env::Default(), JournalDir(config_.work_dir()));
  Update update;
  bool end_of_journal = false;
  FileJournalReader reader(Env::Default(), JournalDir(config_.work_dir()));
  