Preloads revenue currencies for a query.

Assumptions and business logic:

1. Goals are already filtered according to query filters and dimensions
2. If there's a single currency involved, return map containing the default
3. If there's a breakdown by event:goal we return all the relevant currencies as a map
4. If filtering by multiple different currencies without event:goal breakdown empty map is returned
5. If user has no access or preloading is not needed, empty map is returned

The resulting data structure is attached to a Query and used below in format_revenue_metric/3.