Prototype distributed actor handles (#1137)

* Add actor handle ID to the task spec

* Local scheduler dispatches actor tasks according to a task counter per handle

* Fix python test

* Allow passing actor handles into tasks. Not completely working yet. Also this is very messy.

* Fixes, should be roughly working now.

* Refactor actor handle wrapper

* Fix __init__ tests

* Terminate actor when the original handle goes out of scope

* TODO and a couple test cases

* Make tests for unsupported cases

* Fix Python mode tests

* Linting.

* Cache actor definitions that occur before ray.init() is called.

* Fix export actor class

* Deterministically compute actor handle ID

* Fix __getattribute__

* Fix string encoding for python3

* doc

* Add comment and assertion.

python/ray/global_scheduler/test/test.py

@@ -170,6 +170,7 @@ class TestGlobalScheduler(unittest.TestCase):
         task2 = local_scheduler.Task(random_driver_id(), random_function_id(),
                                      [random_object_id()], 0, random_task_id(),
                                      0, local_scheduler.ObjectID(NIL_ACTOR_ID),
+                                     local_scheduler.ObjectID(NIL_ACTOR_ID),
                                      0, 0, [1.0, 2.0, 0.0])
         self.assertEqual(task2.required_resources(), [1.0, 2.0, 0.0])