Fetch children depending on language overlay mode

We have to make sure getRecordOverlay will only be used for scenarios
using language overlays for content elements.

Otherwise children must be fetched based on the origin container ID

Additionally: Some changes according to TYPO3 62. performance tuning

Change-Id: I9ffe1e6d1f59a74578b3cdf03ce87bac59d5f659
Resolves: #55164
Reviewed-on: https://review.typo3.org/26957
Reviewed-by: Jo Hasenau
Tested-by: Jo Hasenau