Almost every specialization of ItemRepositoryFor class template defines
a public static member function init() with the same trivial definition.
Replace the duplicated init() with LockedItemRepository::initialize().

The refactoring reveals that ItemRepositoryFor<IndexedString>::init()
was unused. init() was also missing from item repositories for
IndexedIdentifier, IndexedQualifiedIdentifier and
IndexedInstantiationInformation. I don't know if this lack of
initialization of Indexed* repositories is fine or a potential bug.