Implement V6 Memfile BLQ query functions

This issue covers Bulk Lease Query task 1.3.c

    1.3 Memfile Lease Mgr
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        c. Implement new query functions

    ...

as described in design doc and defined in LeaseMgr under #2571 (closed)

https://gitlab.isc.org/isc-projects/kea/-/wikis/Designs/Bulk-LeaseQuery-Design#query-data-requirements

It depends on #2584 (closed).