- Executor is a superclass of ExecutorService, so this is arguably a little
more flexible.
- It removes the need to use null for direct execution, because Runnable::run
is a direct executor that can be trivially used instead.
- Removing the error-prone "cannot be a direct executor" restriction in the
parallel version of SegmentDownloader requires not relying on the futures
returned from ExecutorService.submit() anyway.
Issue: #5978
PiperOrigin-RevId: 308586620
