=act #3623 taking care of refs before an unsubscriber is set

During initializing with an unsubscriber, it gets all previously
subscribed refs for this eventStream, and can watch them.
Because we cannot easily get everyone who's already subscribed, changing
the unsubscriber is not supported - init can be called only once (as the
name implies)

Initialization is performed only once during an actor system's `_start`.