If an offerGroup already has a subscription, the lib must forbid explicit start dates as it cannot handle a list of interrupted subscriptions

[ziedHamdi]

IUserCredits.offers holds a list of active offerGroups with a start and expiry date each. When a customer purchases an offer from 1st to 2nd january, then buys in the same offerGroup another with a delayed start date from 30 to 31 december, the order would be saved as from 1st january to 31 december.

So delayed orders must be forbidden at creation IService.createOrder and payment time IService.payOrder if an order from the same offerGroup is active. Otherwise, each offerGroup in orders should contain a list of future active subscriptions.