Optimize ServiceReference.isAssignableTo

The ServiceReference.isAssignableTo(Bundle bundle, String className) does some complicated check to see if the service is assignable. This includes trying to load classes and catching CNFE to see if providers and/or requesters have access to the given class.

As that is done quite a bit (it is used in service event dispatching and service lookup) it would be good to optimize this to either not throw CNFE during the lookup or maybe at least cache the results to not have them be done all the time.