Search reindexing causes memory exhaustion on some systems

Search reindexing processes will consume all available memory and fail on some systems. This has been tracked to how settings are managed via a delegate ModelSettings instance in ca_list_items. For some reason, on some (and only some) PHP 7.4 systems, the destructor is never called and memory not freed up for these instances. Avoiding instantiation of a very large number of ModelSettings instances appears to be the only solution.