Opcache tuning

Hey Dmitry, I think it would be helpful if we added two tuning status entries. The first is a "restart_count" which lets you know how many times the cache has restarted since server start. And the second, since there are multiple triggers for a cache restart, would be a "last_restart_reason" which specifies which limit triggered the last restart. Both should be easy to do as far as I can tell and it would help people figure out if they need to allocate more memory to the cache or allocate more keys. -Rasmus