Remove counter_payout from RenewOffer #149
Description
The counterparty payout for the Contract that is being replaced by the renew protocol can be computed by calculating the difference between the collateral of the to-be-replaced Contract and the ContractInput in the RenewOffer. This value can then be used to compute the implicit PNL of the replaced Contract.
As such we don't need/want to keep around the extra counter_payout argument and field throughout the renew protocol.