Figure out how to properly add a time delta per-player

[brandonsturgeon]

Right now if a player joins 1 second before the batch update, they'll get an extra updateInterval - 1 seconds.

We could set their duration to now - joined but I like the idea of it always being additive instead of definitive.

Once we fix this, we can also fix the issue where if the timer stops then starts, some players would get way more time than they earned (if they joined well after the timer stopped) because the timer will always add now - lastUpdate seconds to every player.