func (c *Client) Increment(key string, delta uint64) (newValue uint64, err error) {
	return c.incrDecr(cmdIncr, key, delta)
}
func (c *Client) Increment(key string, delta uint64) (newValue uint64, err error) {
	return c.incrDecr(cmdIncr, key, delta)
}
// Increment atomically increments key by delta. The return value is
// the new value after being incremented or an error. If the value
// didn't exist in memcached the error is ErrCacheMiss. The value in
// memcached must be an decimal number, or an error will be returned.
// On 64-bit overflow, the new value wraps around.