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:

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

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