Enable function_descriptor in backend to replace the function_id (#3028)

python/ray/import_thread.py

@@ -98,7 +98,7 @@ class ImportThread(object):
             # Keep track of the fact that this actor class has been
             # exported so that we know it is safe to turn this worker
             # into an actor of that class.
-            self.worker.imported_actor_classes.add(key)
+            self.worker.function_actor_manager.imported_actor_classes.add(key)
         # TODO(rkn): We may need to bring back the case of
         # fetching actor classes here.
         else: