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.
yyforyongyu