[TASK] Lower memory use in ArrayUtil->mergeRecursiveWithOverrule

Iterate on the original array instead of an array_keys()-copy in
ArrayUtil->mergeRecursiveWithOverrule which lowers memory usage and
enhances performance.

Resolves: #63812
Releases: master, 6.2
Change-Id: I71acc9e1e730f1e782d0e74acd45637ce1b9f08e
Reviewed-on: http://review.typo3.org/35351
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>