Fill driver id into actor notification when finishing assigned task. (#3080)
Fill driver id into actor notification when finishing assigned task. (#3080)

## What do these changes do?
Fill driver id into actor notification when finishing assigned task.
Also it improves codes.
ujvl/ray-ng,stephanie-wang/ray,pcmoritz/ray-1,ray-project/ray,pcmoritz/ray-1,ray-project/ray,richardliaw/ray,atumanov/ray,robertnishihara/ray,pcmoritz/ray-1,pcmoritz/ray-1,atumanov/ray,atumanov/ray,ujvl/ray-ng,stephanie-wang/ray,stephanie-wang/ray,atumanov/ray,robertnishihara/ray,robertnishihara/ray,ray-project/ray,atumanov/ray,pcmoritz/ray-1,pcmoritz/ray-1,stephanie-wang/ray,stephanie-wang/ray,ujvl/ray-ng,atumanov/ray,stephanie-wang/ray,atumanov/ray,stephanie-wang/ray,richardliaw/ray,atumanov/ray,ray-project/ray,ujvl/ray-ng,richardliaw/ray,robertnishihara/ray,pcmoritz/ray-1,richardliaw/ray,stephanie-wang/ray,robertnishihara/ray,robertnishihara/ray,ray-project/ray,richardliaw/ray,richardliaw/ray,robertnishihara/ray,robertnishihara/ray,ray-project/ray,richardliaw/ray,pcmoritz/ray-1,ray-project/ray,ray-project/ray,stephanie-wang/ray,richardliaw/ray,robertnishihara/ray