Companion

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.

Why 253 and not 250 since feerate-per-kw should be feerate-per-kvb / 4 and the minimum relay fee rate is 1000 satoshi/kvb (see [MinimumRelayFeeRate])?

Minimum relay fee rate in satoshi per kilo-vbyte (taken from Bitcoin Core). Note that Bitcoin Core uses a virtual size and not the actual size in bytes: see [MinimumFeeratePerKw] below.