Ray scheduler spillback plumbing + mechanism (#1362)

* spillback mechanism and plumbing : adding spillback counter + timestamp

* linting fix

* documentation

* Fix argument name.

python/ray/monitor.py

@@ -198,7 +198,8 @@ class Monitor(object):
                 key, "RAY.TASK_TABLE_UPDATE",
                 hex_to_binary(task_id),
                 ray.experimental.state.TASK_STATUS_LOST, NIL_ID,
-                task["ExecutionDependenciesString"])
+                task["ExecutionDependenciesString"],
+                task["SpillbackCount"])
             if ok != b"OK":
                 log.warn("Failed to update lost task for dead scheduler.")
             num_tasks_updated += 1