Make SessionHolder publicly accessible for external resource management

[rstoyanchev]

SessionHolder has become package private in main for 7.0, which leaves no obvious alternative to update code like HibernateFlowExecutionListener in Web Flow, which calls bindResource of TransactionSynchronizationManager.

Using the base class EntityManagerHolder also doesn't work because HibernateTransactionManager expects SessionHolder in doGetTransaction and throws a ClassCastException.