Exposes a mutable array on items a player should keep on death.
This allows a cleaner method to implement "Keep certain items on death"
than how plugins currently do it in that it never removes them in first
place, so its safe if the player logs out/server is shutdown before respawn.
Example Usage: https://gist.github.com/aikar/5bb202de6057a051a950ce1f29feb0b4
