Implements SubscriptionHandle returned by MemoryListener::subscribe() to unsubscribe from memory via SubscriptionHandle::release() or MemoryListener::unsubscribe(SubscriptionHandle handle) at a later point in time. Further implements ScopedSubscriptionHandle which is move-assignable via a SubscriptionHandle and automatically unsubscribes when running out of scope.
Needed for sw/armarx/skills/speech!4 (merged) (see sw/armarx/skills/speech!4 (comment 64668))