AsyncNotificationCenter asynchronous notifying

[aleks-f]

Is your feature request related to a problem? Please describe.
AsyncNotificationCenter currently receives notifications asynchronously. However, it notifies observers synchronously, which means that processing is as fast as the slowest handler; much worse, a blocked observer notification handler stops all subsequent pending notifications.

Describe the solution you'd like
Add capability to notify asynchronously. This is best added as a property of Notification (or introducing AsyncNotification class as a child of Notification). AsyncNotificationCenter can then use ActiveMethod for spawning notifications marked as asynchronous and keep the list of pending results for housekeeping. Sync notification mode can remain default, so that the current behavior is not broken.

Describe alternatives you've considered
None, AsyncNotificationCenter is async/sync asymmetric and that should be changed.