Allow for caching conversion rates

Overview

This PR incorporates caching capacity to the currency converter extension, so that users make less calls to external APIs.

Before

Previously, this extension made a call to an external API once per transaction to look up the exchange rate.

After

Services such as Currency layer and Api layer, have changed their business model and offer a limited number of calls per month for the different plans. In order to keep costs down for clients who use those services and this extension, these changes store the exchange rates in the database so that if caching is enabled from the Currency Converter settings page, then the rates are looked up for the first transaction of that day and reused for the rest of that day's transactions.