Scheduled orders cannot be discontinued

When discontinuing an order, the API checks if it's active, in 2.0 the logic in Order.isActive() was changed to first check if an order is started but Order.started() returns false for a scheduled order so discontinuing it becomes impossible.

commit where the bug was introduced: https://github.com/openmrs/openmrs-core/commit/d65825b685453515f55e30504e9cd92ebaa3f759
see removal of deprecated isFuture() and its replacement with isStarted(checkDate) in isActive(checkDate)