Use XRay backend by default. (#3020)

* Use XRay backend by default.

* Remove irrelevant valgrind tests.

* Fix

* Move tests around.

* Fix

* Fix test

* Fix test.

* String/unicode fix.

* Fix test

* Fix unicode issue.

* Minor changes

* Fix bug in test_global_state.py.

* Fix test.

* Linting

* Try arrow change and other object manager changes.

* Use newer plasma client API

* Small updates

* Revert plasma client api change.

* Update

* Update arrow and allow SendObjectHeaders to fail.

* Update arrow

* Update python/ray/experimental/state.py

Co-Authored-By: robertnishihara <robertnishihara@gmail.com>

* Address comments.

python/ray/function_manager.py

@@ -5,6 +5,7 @@ from __future__ import print_function
 import hashlib
 import inspect
 import json
+import sys
 import time
 import traceback
 from collections import (
@@ -342,6 +343,14 @@ class FunctionActorManager(object):
         checkpoint_interval = int(checkpoint_interval)
         actor_method_names = json.loads(decode(actor_method_names))
 
+        # In Python 2, json loads strings as unicode, so convert them back to
+        # strings.
+        if sys.version_info < (3, 0):
+            actor_method_names = [
+                method_name.encode("ascii")
+                for method_name in actor_method_names
+            ]
+
         # Create a temporary actor with some temporary methods so that if
         # the actor fails to be unpickled, the temporary actor can be used
         # (just to produce error messages and to prevent the driver from