Fixed a bug so that a versioned entity with a oneToOne id can be created

- Fixed the basic entity persister so that versioned OneToOne entities can be created
- Created an IdentifierConverter utility class
- Moved the logic for the flatten identifier method into the new utility class
- Replaced calls for private flattenIdentifier to use new utility
- Added appropriate unit tests
I don't think I know enough about the internals to answer this. The method already existed in UnitOfWork, I just moved it into this class for better re-use