What if the ledger ignored blocks included via "Recursive EBs" and "Late IBs" that are from older pipeline instances than the EB in the latest RB?

[nfrisby]

(This idea could address some of the questions in #378.)

Suppose:

• The base protocol ledger state records the pipeline index of the EB in the latest applied RB.
• The application of an RB to a ledger state skips any included EBs and/or IBs that are from pipeline stages earlier than the ledger state's recorded pipeline index.

Recall that the "Recursive EB" features requires that the EBs are present if they were already certified. So the EB in each RB cannot omit successful EBs.

Pros:

• The motivating cases for these features (absence of EBs or absence of RBs) would not cause the blocks to be skipped.
• All IBs and EBs that were not skipped would be applied to the ledger state in non-descending slot order.

Cons:

• It's possible that some EBs/IBs included via "Recursive EBs" and/or "Late IBs" are not redundant, even if EBs from the contemporary pipelines were already included on chain. So there's a missed opportunity here for those txs to be included.
• Edit: ah ha. "Recursive EBs" does not ensure the EB includes certified EBs from the preceding two pipeline instances, since those instances are still in the process of voting/diffusing votes when this EB is built. Thus an RB that includes the latest EB it's aware of might be skipping over two successful pipeline instances.
  • A possible mitigation: an RB can (and must, if possible) include up to three adjacent EBs and their certificates.
    • If that's too big, maybe this is enough motivation to compress VoteSend+VoteRecv (back) down to a single slice?
    • Hmm. https://github.com/input-output-hk/ouroboros-leios/blob/main/docs/cost-estimate/03-storage.md says ~7 kibibytes for a cert, and EBs also can't be too big. So maybe 3 EBs is fine as-is?
  • A possible mitigation: skip EBs and IBs that are more than two pipeline instances older than the latest RB's EB. This still admits some out-of-orderness, but it's much more bounded.
  • A possible mitigation: note that "Late IBs" already includes those skipped iteration's IBs.