[4.2] File Store Increment/Decrement

This behaves in the same way that other cache drivers do. If the cache key doesn't exist, it will update it from 0, setting the expiry time long in the future or "forever". The only inconsistency here is that it will also ignore the expiry time if it was previously set. I assume this problem was the reason this functionality was not originally implemented. Surely an implementation despite this floor is better than no implementation?
I would not have created a new `getInt` function
```php
$int = (int) $this->get($key) + $value;
// (int) null => 0
```