UUIDCatalog::iterator::uuid() should not call repositionIfNeeded

UUIDCatalog::iterator::uuid() should simply return the _uuid even if this uuid is dropped after the function returns. It's the caller's job to double check whether this uuid still exists. If UUIDCatalog::iterator::uuid() does the checking itself (by calling repositionIfNeeded), the result might still be outdated after it returns.