Fixed a bug so that a versioned entity with a OneToOne defined as an id can be created

Changed the visibility of UnitOfWork::flattenIdentifier for use in BasicEntityPersister.

In BasicEntityPersister::fetchVersionValue I have added a line to flatten the id's before fetching the version value.

Without this change, an entity with a OneToOne relationship defined as an Id, with a Version property, could not be created. Rather than trying to use the ID in the query to select the version, the related entity was being passed.
@MajorCaiger the UoW could re-use that utility as well eventually...