tx-sub: added mempoolLastTicket to MempoolSnapshot

[crocodile-dentist]

Description

This is just to expose what consensus already tracks so we don't have to traverse the list of transactions to find the last tx index/ticket. This will also come in handy in dmq node to client code.

Checklist

Quality

• Commit sequence makes sense and have useful messages, see ref.
• New tests are added and existing tests are updated.
• Self-reviewed the PR.

Maintenance

• Linked an issue or added the PR to the current sprint of ouroboros-network project.
• Added labels.
• Updated changelog files.
• The documentation has been properly updated, see ref.

[coot]

At least in main branch, we'll need to subtract 1, as the indexes are 0-based, but not in this branch. So this is not the reason why the test fails.

Should we actually return Nothing if the list is empty?

Suggested change

	mempoolLastTicket = Just $ Seq.length seq
	mempoolLastTicket = if Seq.length seq > 0 then
	then Seq.length seq - 1
	else Nothing

[crocodile-dentist]

I've tried this and it didn't work. I've also added a counter to the mempool state because perhaps relying on the length is not correct, but that also didn't work. I will have to look at the logs more carefully as I've ran out of obvious simple tweaks.

[crocodile-dentist]

The problem is that this mempoolLastTicket moves the window too much, so we skip tx's. This doesn't matter for tx submission that much where we only ask for a few at a time, but the local dmq protocols will probably transmit more between the peers. We could change mempoolTxIdsAfter to mempoolTxIdsSlice to do what we want.

As an aside, I've noticed that the tx submission server could be more eager in requesting tx id's. The server will often make a request for 1 txid when it could ask for all that the protocol allows. For eg, here is an example of an initial request:
TraceTxInboundDecision (TxDecision {txdTxIdsToAcknowledge = NumTxIdsToAck 0, txdTxIdsToRequest = NumTxIdsToReq 1, txdPipelineTxIds = False, txdTxsToRequest = fromList [], txdTxsToMempool = []})
It only asked for one, when it could have requested for the maximum unacknowledged limit.
Later, after receving the single lone tx, it will send another request for one tx id while acknowledging that prior tx id.
TraceTxInboundDecision (TxDecision {txdTxIdsToAcknowledge = NumTxIdsToAck 1, txdTxIdsToRequest = NumTxIdsToReq 1, txdPipelineTxIds = False, txdTxsToRequest = fromList [], txdTxsToMempool [...]=
[edit] nevermind, this is just from the arbitrary instance for tx decision policy.