Optimise cart link

Here's a couple of tweaks to optimise cart_link:

* in `Spree::Core::ControllerHelpers::Order#find_order_by_token_or_user` ensures that the `option[:lock]` defaults to false for both `simple_current_order` and `current_order`
* in `Spree::Core::ControllerHelpers::Order#simple_current_order` if the order isn't found, return a new empty order that is then memoized instead of returning nil - this avoids hitting the DB twice

The really big optimisation we found though was to index the `spree_orders.guest_token` column - is it appropriate to add this to this PR? 
Go ahead and make the change. We're just starting to implement standards, and we know that the existing code doesn't conform to them. We'll just try and improve it a little bit at a time.