[core] current scheduler

- Adds a way to set the current scheduler on the thread to be used whenever a mailbox is created that needs to reply on this thread

platform/default/thread_local.cpp

@@ -58,8 +58,8 @@ void ThreadLocal<T>::set(T* ptr) {
     }
 }
 
-template class ThreadLocal<RunLoop>;
 template class ThreadLocal<BackendScope>;
+template class ThreadLocal<Scheduler>;
 template class ThreadLocal<int>; // For unit tests
 
 } // namespace util