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
Yeah, I was unsure what to call it, and wasn't sure whether anything else could live in here, so I didn't want to name it too specifically, I can rename it if you prefer?