Feature description

I think it'd be really helpful to have a feature where the locks expire after a configurable amount of time of the owner being offline.

I believe this could be checked when a player tries to access a locked box.

I think it'd be helpful for things like #133 where players just want access to items from players who quit.

Thoughts: I'm a bit new to Minecraft plugins, but if there's a way to hook into something like onLogout and onLogin to update something like lastSeen on a particular player, that'd be a good way to have a reliable variable tracking when an owner was last seen.