DiscountCode with validity period

We have introduced validity ranges on Discount Codes for limiting the time range in which a code can be applied to a cart. Setting a validity can also help to manage campaigns which are promoted via discount codes, also considering that one code may contain several different cart discounts with varying validity dates.

• [API] Added validFrom and validUntil fields in a DiscountCode.
• [API] Added "setValidFrom" and "setValidUntil" update actions for a DiscountCode.
• [API] Added DiscountCodeState NotValid and ApplicationStoppedByPreviousDiscount.