Implement a first pass at actors in the API. (#242)

* Implement actor field for tasks

* Implement actor management in local scheduler.

* initial python frontend for actors

* import actors on worker

* IPython code completion and tests

* prepare creating actors through local schedulers

* add actor id to PyTask

* submit actor calls to local scheduler

* starting to integrate

* simple fix

* Fixes from rebasing.

* more work on python actors

* Improve local scheduler actor handlers.

* Pass actor ID to local scheduler when connecting a client.

* first working version of actors

* fixing actors

* fix creating two copies of the same actor

* fix actors

* remove sleep

* get rid of export synchronization

* update

* insert actor methods into the queue in the right order

* remove print statements

* make it compile again after rebase

* Minor updates.

* fix python actor ids

* Pass actor_id to start_worker.

* add test

* Minor changes.

* Update actor tests.

* Temporary plan for import counter.

* Temporarily fix import counters.

* Fix some tests.

* Fixes.

* Make actor creation non-blocking.

* Fix test?

* Fix actors on Python 2.

* fix rare case.

* Fix python 2 test.

* More tests.

* Small fixes.

* Linting.

* Revert tensorflow version to 0.12.0 temporarily.

* Small fix.

* Enhance inheritance test.

src/common/state/actor_notification_table.c

@@ -0,0 +1,16 @@
+#include "actor_notification_table.h"
+#include "redis.h"
+
+void actor_notification_table_subscribe(
+    db_handle *db_handle,
+    actor_notification_table_subscribe_callback subscribe_callback,
+    void *subscribe_context,
+    retry_info *retry) {
+  actor_notification_table_subscribe_data *sub_data =
+      malloc(sizeof(actor_notification_table_subscribe_data));
+  sub_data->subscribe_callback = subscribe_callback;
+  sub_data->subscribe_context = subscribe_context;
+
+  init_table_callback(db_handle, NIL_ID, __func__, sub_data, retry, NULL,
+                      redis_actor_notification_table_subscribe, NULL);
+}