Use asset values instead of asset prices in oracle calls

[alcueca]

Refactor to integrate with oracles using asset value terms instead of asset price terms.

Something is off in the liquidations, needs some debugging.

Also, the liquidations now do two oracle calls (one for _isHealthy and another to determine the liquidation), that is probably a non-starter due to gas reasons. If the liquidations can be refactored to work on position proportions instead of price then we wouldn't need the second oracle call.

[MathisGD]

Thank you for your POC. I'm not in favor of continuing its development, as I don't find that it makes the code so much clearer. Also, as you mentioned in #535, we loose an optimization avoiding to call the oracle twice.

One advantage that it has, that I have not see mentioned, is that it enables complex oracles returning a different price depending on the amount (to reflect the market). But it's not even sure that such oracle will ever exist.