HtlcTx

It's important to note that htlc transactions with the default commitment format are not actually replaceable: only anchor outputs htlc transactions are replaceable. We should have used different types for these different kinds of htlc transactions, but we introduced that before implementing the replacement strategy. Unfortunately, if we wanted to change that, we would have to update the codecs and implement a migration of channel data, which isn't trivial, so we chose to temporarily live with that inconsistency (and have the transaction replacement logic abort when non-anchor outputs htlc transactions are provided). Ideally, we'd like to implement a dynamic commitment format upgrade mechanism and depreciate the pre-anchor outputs format soon, which will get rid of this inconsistency. The next time we introduce a new type of commitment, we should avoid repeating that mistake and define separate types right from the start.