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.

test/multi_node_test.py

@@ -202,7 +202,7 @@ def ray_start_head_with_resources():
 
 
 @pytest.mark.skipif(
-    os.environ.get("RAY_USE_XRAY") != "1",
+    os.environ.get("RAY_USE_XRAY") == "0",
     reason="This test only works with xray.")
 def test_drivers_release_resources(ray_start_head_with_resources):
     redis_address = ray_start_head_with_resources