Refactor code about ray.ObjectID. (#3674)

* Refactor code about ray.ObjectID.

* remove from_random and use nil_id instead of constructor

* remove id() in hash

* Lint and fix

* Change driver id to ObjectID

* Replace binary_to_hex(ObjectID.id()) to ObjectID.hex()

test/failure_test.py

@@ -12,10 +12,11 @@ import threading
 import time
 
 import ray.ray_constants as ray_constants
-import ray.test.cluster_utils
 from ray.utils import _random_string
 import pytest
 
+from ray.test.cluster_utils import Cluster
+
 
 def relevant_errors(error_type):
     return [info for info in ray.error_info() if info["type"] == error_type]
@@ -620,7 +621,7 @@ def test_warning_for_too_many_nested_tasks(shutdown_only):
 @pytest.fixture
 def ray_start_two_nodes():
     # Start the Ray processes.
-    cluster = ray.test.cluster_utils.Cluster()
+    cluster = Cluster()
     for _ in range(2):
         cluster.add_node(
             num_cpus=0,
@@ -674,6 +675,8 @@ def test_raylet_crash_when_get(ray_start_regular):
 
     thread = threading.Thread(target=sleep_to_kill_raylet)
     thread.start()
-    with pytest.raises(Exception, match=r".*raylet client may be closed.*"):
+    with pytest.raises(
+            ray.raylet.RayCommonError,
+            match=r".*raylet client may be closed.*"):
         ray.get(nonexistent_id)
     thread.join()