CIP-65: Updating the score definition for buy orders

We believe that the solver compensation is the core part of CoW Protocol, and this is a fundamental problem that requires an immediate solution. The proposed approach, which adjusts the definition of surplus from being based on limit sell amount to being based on the actual executed buy amount, would address the issue of inflated reward for solvers. There may be still ways to intentionally increase the gap between limit buy amount and executed buy amount, but that would not lead to higher scoring or rewards under the new scroing mechanism.