Add flood protection to cron

This query from acitivity_cron is showing up in slow query logs on a client site.

 $uid_sql = "SELECT uid, min(created) as min, count(aid) as count FROM {activity} GROUP BY uid HAVING min < %d AND count > %d";

I can't yet find a way to optimize the query itself, however it seems like it'd be easy enough to use core's flood protection to prevent it from running every cron run - i.e. just run it once per day by default.

Should be able to get a patch for this going later.