Add support for Platform Modules and more generally Extensions to provide new Hibernate mappings

I'd like to cover 2 use cases at once:

• Ability for a platform module to contribute Hibernate mappings (static)
• Ability for an extension installed at runtime to contribute Hibernate mappings (dynamic)

The solution is to:

• Step 1: Create a new Hibernate Configuration instance with all the mappings (old + new)
• Step 2: Create a new SessionFactory by calling myconf.buildSessionFactory()
• Step 3: Modify all code so that no code caches the SessionFactory and instead always call an XWiki component to retrieve the Session Factory

I'd like to reorganize a bit the work stared by Artem with the HibernateSessionFactory role.