DATACMNS-609 - Improved bean definition registration for repository configuration.

The bean definitions that were registered for a repository configuration setup we registered once for every usage of the repository configuration element (annotation or XML). This caused multiple registrations of the very same bean definition which - as in case of the RepositoryInterfaceAwareBeanPostProcessor - apparently leads to performance problems in the container startup. Feedback for the latter claim is already asked for but we improved the registration setup nonetheless.

Introduced a registerIfNotAlreadyRegistered(…) method on RepositoryConfigurationExtensionSupport to allow easy registration of to-b-registered-once-and-only-once beans. We now hint towards the newly introduced method in registerWithSourceAndGeneratedBeanName(…).