Fix tracking of collected value across pathfinding iterations

Fix tracking of collected value across pathfinding iterations

If we end up "paying" for an `htlc_minimum_msat` with fees, we
increment `already_collected_value_msat` by more than the amount
of the path that we collected (who's `value_contribution_msat` is
higher than the total payment amount, despite having been reduced
down to the payment amount).

This throws off our total value collection target, though in the
coming commit(s) it would also throw off our path selection
calculations.