NEMO-2057 - timeout on checkout/payment with large # configured tax rates

This performance enhancement fixes page timeouts on the checkout payment step when a large number of tax rates are configured. 

With 54 tax rates configured, dropped average page response (locally) from ~20s (450 sql, dynamic by # tax rates) to ~2s (136 sql, linear to # tax rates). A 90% improvement and safeguard agains larger datasets.