Improve cart validation performance

Nope... There's a comment explainig above that the orders are likely to be loaded anyway. Sorry, but this isn't going to change.

@jsacksick

In the comment, the dev pointed out that he assumes. We just need to call getCartIds without needing to load it.

// Getting the cart data and validating the cart IDs received from the
// session requires loading the entities. This is a performance hit, but
// it's assumed that these entities would be loaded at one point anyway.