Add `receivedAmount` in Order struct

[hilmarx]

For some use cases, it would be quite helpful to know on-chain how many buyTokens were received from a given order, as the Order Struct currently only stores the usedAmount.

Maybe something like that:

struct Order {
        uint16 buyToken;
        uint16 sellToken;
        uint32 validFrom; // order is valid from auction collection period: validFrom inclusive
        uint32 validUntil; // order is valid till auction collection period: validUntil inclusive
        uint128 priceNumerator;
        uint128 priceDenominator;
        uint128 usedAmount; // remainingAmount = priceDenominator - usedAmount
        uint128 receivedAmount; 
}

An example of such use case would be if someone would like to know on-chain how much of the buyTokens were sold and hence can safely be requested to be withdrawn at a later time without potentially overriding another concurrent order.