func IncrementExisting(c context.Context, key string, delta int64) (newValue uint64, err error) {
	return incr(c, key, delta, nil)
}
func IncrementExisting(c context.Context, key string, delta int64) (newValue uint64, err error) {
	return incr(c, key, delta, nil)
}
// IncrementExisting works like Increment but assumes that the key
// already exists in memcache and doesn't take an initial value.
// IncrementExisting can save work if calculating the initial value is
// expensive.
// An error is returned if the specified item can not be found.