Sequential Swaps L1 Transaction Batching

[cryptoquick]

I believe it's possible to batch the sequential swaps PSBTs into a single transaction to save on fees. So, if the previous sequential swap transaction has not yet been mined, it might be possible to RBF the RGB21 change output and create a new transaction with another swap embedded. This requires keeping the old PSBTs around in case they're needed for tx batching. This should cut down on fee costs by up to 50%. This could then keep the fee savings on the RGB21 change output.

Depends #413