Use a CHM for StructureTemplate.Pallete cache

fixes a CME due to this collection being shared across threads

This patch was in 1.16.5 for a good while and noticed somebody
getting impacted by the same thing

patches/server/0860-Use-a-CHM-for-StructureTemplate.Pallete-cache.patch

@@ -0,0 +1,20 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Shane Freeder <theboyetronic@gmail.com>
+Date: Mon, 12 Jul 2021 12:28:29 +0100
+Subject: [PATCH] Use a CHM for StructureTemplate.Pallete cache
+
+fixes a CME due to this collection being shared across threads
+
+diff --git a/src/main/java/net/minecraft/world/level/levelgen/structure/templatesystem/StructureTemplate.java b/src/main/java/net/minecraft/world/level/levelgen/structure/templatesystem/StructureTemplate.java
+index ef8dd3fa4f7ac8f85ae508999264850659bf9606..ac80779956ac2f8aa83166d3d2282352ebe1901d 100644
+--- a/src/main/java/net/minecraft/world/level/levelgen/structure/templatesystem/StructureTemplate.java
++++ b/src/main/java/net/minecraft/world/level/levelgen/structure/templatesystem/StructureTemplate.java
+@@ -831,7 +831,7 @@ public class StructureTemplate {
+     public static final class Palette {
+ 
+         private final List<StructureTemplate.StructureBlockInfo> blocks;
+-        private final Map<Block, List<StructureTemplate.StructureBlockInfo>> cache = Maps.newHashMap();
++        private final Map<Block, List<StructureTemplate.StructureBlockInfo>> cache = Maps.newConcurrentMap(); // Paper
+ 
+         Palette(List<StructureTemplate.StructureBlockInfo> infos) {
+             this.blocks = infos;