EZP-20491 improvement : limitation of memory usage

bug https://jira.ez.no/browse/EZP-20491 had already been fixed by limiting the number of objects to be treated...

but it can be improved by optimizing PHP code, to avoid using too much memory within the loops.

A simple memory optimization is to always use asObjects=false inside a huge PHP loop,
On our projects, the application of this patch decreased a lot the memory usage (divide by 3), and work like a charm with 200k objects