Fix CGLIB memory leak for method injection

This commit continues the work for fixing memory leaks resulting from
CGLIB subclass generation for beans relying on method injection.

- Set proxy callbacks on the CGLIB Factory (i.e., the instance) instead
  of in the generated subclass (i.e., via the Enhancer).

- Convert private inner classes in CglibSubclassingInstantiationStrategy
  to private static classes in order to avoid unnecessary coupling to
  classes generated using CGLIB.

- Tidy up XmlBeanFactoryTests.

- Update logic in serializableMethodReplacerAndSuperclass() so that it
  finally aligns with the decision made for SPR-356.

Issue: SPR-10785, SPR-356