Improve support for @Conditional on @Configuration

Introduce new ConfigurationCondition interface allowing more
fine-grained control for @Conditional when used with @Configuration
beans.

Primarily added so that the evaluation of conditions that inspect bean
definitions can be deferred until all @Configuration classes have been
parsed.

Issue: SPR-10534
Make ConfClassPostPro Ordered.HIGHEST_PRECEDENCE

Make ConfigurationClassPostProcessor Ordered.HIGHEST_PRECEDENCE in
case several BeanDefinitionRegistryPostProcessors are registered.

Issue: SPR-10645
Polish Javadoc