SessionFactoryBuilderSupport implementations create DisposableBean
proxies for SessionFactory objects created using #buildSessionFactory.
Prior to this change, these proxies create problems when working agaist
SessionFactoryUtils.getDataSource(SessionFactory), because this method
expects the given SessionFactory to implement Hibernate's
SessionFactoryImplementor interface (which the stock SessionFactoryImpl
does).
With this change, the DisposableBean proxies created by SFBuilders
now also implement SessionFactoryImplementor to satisfy this and
probably other such cases.
Issue: SPR-8469