Leios requirements for the transaction Mempool

[nfrisby]

This unanswered Leios question is relevant to how the Mempool needs to change.

For example, perhaps the answer is: the IB will have the RB-pointer field, and there's no limit on its age. Then today's transaction Mempool could stay the same, and the node minting an IB would simply flush it into that IB and record the "last applied block" of isLedgerState as the RB-pointer field of that IB.

But currently it's undecided, and so might need something else. For example, if the RB can't be too young, then the Mempool's syncing command should be using the youngest old-enough RB instead of simply its tip, etc.