cache.go
func (c *cache) IncrementInt(k string, n int) (int, error) {
	c.mu.Lock()
	v, found := c.items[k]
	if !found || v.Expired() {
		c.mu.Unlock()
		return 0, fmt.Errorf("Item %s not found", k)
	}
	rv, ok := v.Object.(int)
	if !ok {
		c.mu.Unlock()
		return 0, fmt.Errorf("The value for %s is not an int", k)
	}
	nv := rv + n
	v.Object = nv
	c.items[k] = v
	c.mu.Unlock()
	return nv, nil
}
func (c *cache) IncrementInt(k string, n int) (int, error) {
	c.mu.Lock()
	v, found := c.items[k]
	if !found || v.Expired() {
		c.mu.Unlock()
		return 0, fmt.Errorf("Item %s not found", k)
	}
	rv, ok := v.Object.(int)
	if !ok {
		c.mu.Unlock()
		return 0, fmt.Errorf("The value for %s is not an int", k)
	}
	nv := rv + n
	v.Object = nv
	c.items[k] = v
	c.mu.Unlock()
	return nv, nil
}
// Increment an item of type int by n. Returns an error if the item's value is
// not an int, or if it was not found. If there is no error, the incremented
// value is returned.