Respect `wp_suspend_cache_addition()`

As indicated in a [recent trac ticket](https://core.trac.wordpress.org/ticket/27538), most of the 3rd party object caches do not respect core's `wp_suspend_cache_addition()` function. As the name suggests, if this function is called, new values should not be added to the cache. The core `WP_Object_Cache` [respects this method](https://github.com/WordPress/WordPress/blob/3.8.1/wp-includes/cache.php#L324-L325) (but oddly, only for `add`, not for `set`). 

At a minimum, this object cache needs to respect the suspension for adds. It will prevent against the unfortunate widget "leakage" issues.