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/ray_constants.py

@@ -5,8 +5,6 @@ from __future__ import print_function
 
 import os
 
-from ray.raylet import ObjectID
-
 
 def env_integer(key, default):
     if key in os.environ:
@@ -15,8 +13,6 @@ def env_integer(key, default):
 
 
 ID_SIZE = 20
-NIL_JOB_ID = ObjectID(ID_SIZE * b"\xff")
-NIL_FUNCTION_ID = NIL_JOB_ID
 
 # The default maximum number of bytes to allocate to the object store unless
 # overridden by the user.