timers: use varbit to track remaining overload time

[Enriath]

At some point, varbits appeared that track how many stat refreshes the player has from overloads. This means we use how antipoisons are tracked and make the overload tracking more accurate, and being able to rely on the varbits to clear the timers means they won't disappear when people do advanced cheese in Chambers! This varbit could also be used to add a brand new Overload Stat Restore timer, and I was going to include it here but couldn't figure out an icon to use.

Could close #12848, though that issue also shows the same issue happens with Prayer Enhances. I have a similar change for those nearly ready, though less information exists on the non (+) tiers, making it more difficult to check without actually making some.