HibernateObjectDao.getByClassId() prone to ClassCastExceptions

The com.atlassian.hibernate.HibernateObjectDao#getByClassId method calls session#get(getPersistentClass(), id).
This is prone to the same sort of ClassCastException as CONF-17657 when accidentally called with an id that actually belongs to a sibling class. This wouldn't be the case if we upgraded to the latest version of hibernate 3, which does not suffer from https://hibernate.onjira.com/browse/HHH-1460