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()

python/ray/import_thread.py

@@ -131,5 +131,5 @@ class ImportThread(object):
                 self.worker,
                 ray_constants.FUNCTION_TO_RUN_PUSH_ERROR,
                 traceback_str,
-                driver_id=driver_id,
+                driver_id=ray.ObjectID(driver_id),
                 data={"name": name})