Add a static cache and context to determine default currency

#2415237: Change site's default currency through a 'commerce_default_currency' alter hook provides the ability to alter the default currency, but in some cases, the module requesting a default currency may need to pass in a related context (provided by og_context, a panel context, etc) to get the correct currency.

The best example is products based on organic groups. A store may have a group of products in Canada or a group in the US. by passing the product via the context, a og specific module can implement the alter with the correct context and provide the default currency for that group.