Improve tax rate reads on checkout

Worked with @RhysStansfield on this one. They were seeing very slow requests, about 20s, on checkout due to the store having hundred tax rates. This patch persists the `zone.kind` on the db and adds up a scope on tax_rate.match so we instantiate less ar objects.

// @JDutil 
What was the reason for this change? It seems a little out of place among the rest of the changes.