func (cd Codec) Add(c context.Context, item *Item) error {
	return singleError(cd.set(c, []*Item{item}, pb.MemcacheSetRequest_ADD))
}
func (cd Codec) Add(c context.Context, item *Item) error {
	return singleError(cd.set(c, []*Item{item}, pb.MemcacheSetRequest_ADD))
}
// Add writes the given item, if no value already exists for its key.
// ErrNotStored is returned if that condition is not met.