func (cd Codec) CompareAndSwap(c context.Context, item *Item) error {
	return singleError(cd.set(c, []*Item{item}, pb.MemcacheSetRequest_CAS))
}
func (cd Codec) CompareAndSwap(c context.Context, item *Item) error {
	return singleError(cd.set(c, []*Item{item}, pb.MemcacheSetRequest_CAS))
}
// CompareAndSwap writes the given item that was previously returned by Get,
// if the value was neither modified or evicted between the Get and the
// CompareAndSwap calls. The item's Key should not change between calls but
// all other item fields may differ.
// ErrCASConflict is returned if the value was modified in between the calls.
// ErrNotStored is returned if the value was evicted in between the calls.