Fee rate units

[tcharding]

The Core RPC API is not consistent with the unit used for fee rates. I can't remember if it changes between versions or between methods but sometimes its BTC/kB and sometimes its satoshis per virtual byte. Fortunately these are different types (f64 and u64 respectively).

However, in the ecosystem, fee rates are also measured in sats per kilo weight unit. Both sats per vb and sats per kwu use u64 - we should make some effort to ensure that Core does not use both and that we are not mixing these up.

Perhaps #58 will help with this if we are checking values returned.