Schdule _delete_old_items using wp-cron

Currently aryo-activity-log, for every insert it does, it also performs a DELETE query to MySQL, something like:

```sql
DELETE FROM `wp_aryo_activity_log` WHERE `hist_time` < ?
```

Now, this is not an issue on not so busy sites, however - let's say you run something like `WP All Import` that can do thousands of changes in a few minutes, it means you issue 1000 `DELETE` queries to MySQL, all doing a table scan (because you also fail to put an index on `hist_time`, this is pretty useless in terms of MySQL performance, specially if you have a few million rows in the table like we do.

My suggestion is to schedule the `_delete_old_items()` function using the wp-cron instead, and pick it up every X hours - after all the history is based on days, so doing a table scan for every insert (aka every action in WP), is extremely overkill.