CommitmentFeerate

Since we're using anchor outputs, we don't need to constantly adjust the feerate of the commitment tx to match current on-chain feerates. We can instead set it to a low-enough value, that still ensures the transaction will relay through the bitcoin network, and then use CPFP to make it confirm. TODO: we should regularly get fee estimations from various sources to ensure this default value remains relay-able, and otherwise raise it dynamically.