This commit adds the ability to manually set the output value for a
loop in swap. This is useful for recovering funds from a loop in swap
that has been fund externaly and was sent to with a different amount
than the one specified in the loop in swap.
As we multiply the feerate by 1000, if the uint16 feerate is set higher
than 66 we would encounter an overflow and as such we could never
have a higher feerate than 65.