[BUGFIX] Getting the identifier for a lazy object fails

Using PersistenceManager::getIdentifierByObject doesn't work for lazy
object since version 6.1, because the function uses the session object
directly. Before the Backend::getIdentifierByObject was used, which
handles lazy objects right.
Solution: use the Backend again.

Resolves: #50548
Releases: 6.2, 6.1
Change-Id: I8381a968f730918b96b6764d3f62bfc5f58509ee
Reviewed-on: https://review.typo3.org/24859
Reviewed-by: Marc Bastian Heinrichs
Tested-by: Marc Bastian Heinrichs