Increase slapd mdb size (Fixes BUGZ-4188)
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This merge request is automatically cherrypicked from !1765 (merged) from source branch https://source.coderefinery.org/ellert/arc/-/tree/increase-mdb-size orginally made by @ellert .
The original MR went to branch: https://source.coderefinery.org/nordugrid/arc/-/tree/master
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Based on patch from Dejan Lesjak
Fixes: BUGZ-4188