Create Memfile V6 cross-reference multi-index container for BLQ

This issue covers Bulk Lease Query task 1.3.b

    1.3 Memfile Lease Mgr
    ...
        b. V6 cross-reference multi-index container
    ...

as discussed in design doc:

https://gitlab.isc.org/isc-projects/kea/-/wikis/Designs/Bulk-LeaseQuery-Design#v6-solutions

This should include creating the container and keeping it populated. It depends on #2573 (closed).