Changes to local scheduler client protocol. (#435)

* Make local scheduler clients receive reply upon registration.

* Fix tests and linting.

src/common/task.h

@@ -16,6 +16,7 @@ struct TaskBuilder;
 #define NIL_TASK_ID NIL_ID
 #define NIL_ACTOR_ID NIL_ID
 #define NIL_FUNCTION_ID NIL_ID
+#define NIL_WORKER_ID NIL_ID
 
 typedef UniqueID FunctionID;