Improve "unfulfilled" scope by adding subquery
Improve "unfulfilled" scope by adding subquery

"SolidusSubscriptions::Installment#unfulfilled" gets the list of
fulfilled installments, loads them ALL into memory, and then performs
a WHERE id NOT IN (...) to get the installments that were not fulfilled,
this commit simplify the scope using a WHERE clause with a subquery.

solidusio-contrib/solidus_subscriptions,solidusio-contrib/solidus_subscriptions,solidusio-contrib/solidus_subscriptions,solidusio-contrib/solidus_subscriptions