Solutions might not be accepted due to too big order amounts

[josojo]

If there are orders in the system, which have huge order amounts in the range of max uint256, these orders might not be accepted in a solution provision.

During the solution calculation, it might happen that the following line throws, due to a calculation overflow: https://github.com/gnosis/dex-contracts/blob/master/contracts/BatchExchange.sol#L743

One example instance is here.

Find a better calculation for the utility in the next version of the smart contract.