We need to make sure we populate all the necessary
witness/nonwitness because for taproot inputs we need the prevout
so we check for it in the hashcash creation.
With this commit we make it possible to just check whether two lists of
public keys can match the given channels and derive the 2-of-2 multisig
channel funding address.
As we multiply the feerate by 1000, if the uint16 feerate is set higher
than 66 we would encounter an overflow and as such we could never
have a higher feerate than 65.