replace SubscriberAccessor & SubscriberAccessorRepository

[DavidBadura]

The SubscriberAccessor class exists for the subscription engine. Its goal was to encapsulate the calling of the subscriber, provide meta information and wrap the subscriber itself. This works very well as long as you don't need to extend anything or want to provide more information for the subscription engine. As soon as you want to add something, it's a BC break. In other words, the current implementation is more of a problem than a help and needs to be replaced.