* feat(AsyncObserver): Improve NotificationCenter speed and usability #4414
* fix(Notification): add missing header
* feat(Any): add checkers for holding nullptr #4447
* feat(NotificationCenter): g++ build and refactoring #4414
* fix(Observer): compile errors on some compilers #4414
* fix(NotificationCenter): compile errors #4414
* chore(ParallelSocketAcceptor): remove unnecessary include and using from header
* feat(AsyncNotificationCenter): add #4414
* test(AsyncNotificationCenter): add mixed observer types to the test #4414
* fix(AsyncNotificationCenter): hangs on program exit #4414
* fix(dev): friend not honored, temporarily make private members public
* fix(AsyncNotificationCenter); remove default #4414
