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 
Why `Spree::Zone.potential_matching_zones(zone)` instead of `zone.potential_matching_zones`