[Proposal] Store all current prices rather than only those of touched orders

[josojo]

The purpose of this proposal is to have a reliable price oracle function which could be improved from its current state by simply storing all prices rather than only those of touched orders. Implementation-wise, it might suffice to carry forward prices from a previous solution if those prices were not included in a future auction (because those particular tokens were not touched). However this is likely easier said than done.