Fix failure of test_free_objects_multi_node (#3481)
Fix failure of test_free_objects_multi_node (#3481)

It is possible that `test_free_objects_multi_node` would fail sometimes. If we run this test 20 times, we may found at least one failure.

The cause is that the test is based on function tasks. One raylet may create more than one worker to execute the tasks. So flush operations may be separated to several workers and not clean all the worker objects held by the plasma client.

In this PR, I change function task to actor tasks, which guarantee all the tasks are executed in one worker of a raylet.
atumanov/ray,pcmoritz/ray-1,robertnishihara/ray,atumanov/ray,robertnishihara/ray,pcmoritz/ray-1,atumanov/ray,robertnishihara/ray,atumanov/ray,robertnishihara/ray,atumanov/ray,richardliaw/ray,ray-project/ray,ujvl/ray-ng,stephanie-wang/ray,ray-project/ray,stephanie-wang/ray,ray-project/ray,ray-project/ray,robertnishihara/ray,pcmoritz/ray-1,pcmoritz/ray-1,pcmoritz/ray-1,richardliaw/ray,richardliaw/ray,richardliaw/ray,robertnishihara/ray,richardliaw/ray,robertnishihara/ray,stephanie-wang/ray,pcmoritz/ray-1,ujvl/ray-ng,ray-project/ray,richardliaw/ray,ray-project/ray,atumanov/ray,ray-project/ray,stephanie-wang/ray,ujvl/ray-ng,robertnishihara/ray,richardliaw/ray,pcmoritz/ray-1,stephanie-wang/ray,atumanov/ray,ujvl/ray-ng,stephanie-wang/ray,stephanie-wang/ray,robertnishihara/ray,richardliaw/ray,stephanie-wang/ray,atumanov/ray,stephanie-wang/ray,ray-project/ray,pcmoritz/ray-1