Fix a bug that causes terminal status update not getting sent.
Fix a bug that causes terminal status update not getting sent.

Specifically:
- Wait 5 seconds after sending the status before the executor terminates itself.
- Make sure task_runner.stop() is always called no matter whether the task succeeds or where it fails.

EasonYi/incubator-cotton,xujyan/cotton,xujyan/cotton,xujyan/cotton,EasonYi/incubator-cotton,EasonYi/incubator-cotton