Since Go 1.22 loop variables are scoped per-iteration, so the
`x := x` / `a, b := a, b` copies inside range/for loops are no longer
needed. This removes the existing redundant copies in test files.
Previously we don't allow confTarget to be 0, which ended up the final
position being never reached. We fix it here by allowing confTarget to
be 0 in case the deadline has already been passed for a given input.
This commit adds a private type `mSatPerKWeight` that expresses a given
fee rate in millisatoshi per kw. This is needed to increase the
precision of the fee function. When sweeping anchor inputs, if using a
deadline delta of over 1000, it's likely the delta will be 0 sat/kw due
to precision.
This commit adds a new interface, `FeeFunction`, to deal with
calculating fee rates. In addition, a simple linear function is
implemented, hence `LinearFeeFunction`, which will be used to calculate
fee rates when bumping fees. Check #4215 for other type of fee functions
that can be implemented.
