Relax single tx input requirements

[t-bast]

In some places of the codebase we relied on the fact that lightning transactions had a single input. That was correct with the standard commitments format, but will not be the case with anchor outputs: 2nd-stage txs (htlc-txs) and 3rd-stage txs (claim-htlc-txs) can be RBF-ed and have any number of inputs and outputs.

[pm47]

This works but may generate warnings in logs. Another strategy would have been to handle delayed input one-by-one, and keep track of the remaining inputs in the BITCOIN_PARENT_TX_CONFIRMED event.

Something like:

case class BITCOIN_PARENT_TX_CONFIRMED(childTx: Transaction, nextCsvTimeouts: Map[ByteVector32, Long]) extends BitcoinEvent

[t-bast]

This works but may generate warnings in logs. Another strategy would have been to handle delayed input one-by-one, and keep track of the remaining inputs in the BITCOIN_PARENT_TX_CONFIRMED event.

TBH this code is very temporary; I'll completely remove and change how we track and publish txs to support regularly bumping tx fees for anchor outputs, so I'd rather go with the simplest approach / the less code changes for now.