PlayerDeathEvent#getItemsToKeep

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

Spigot-Server-Patches/Optimize-ItemStack.isEmpty.patch

@@ -6,7 +6,7 @@ Subject: [PATCH] Optimize ItemStack.isEmpty()
 Remove hashMap lookup every check, simplify code to remove ternary
 
 diff --git a/src/main/java/net/minecraft/server/ItemStack.java b/src/main/java/net/minecraft/server/ItemStack.java
-index 56787ed25c..f71d9ee578 100644
+index e0f782acc..865ff2ee1 100644
 --- a/src/main/java/net/minecraft/server/ItemStack.java
 +++ b/src/main/java/net/minecraft/server/ItemStack.java
 @@ -0,0 +0,0 @@ public final class ItemStack {