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 
Place the . on the previous line, together with the method call receiver.<br>Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.