Use a CHM for StructureTemplate.Pallete cache

fixes a CME due to this collection being shared across threads

paper-server/patches/sources/net/minecraft/world/level/levelgen/structure/templatesystem/StructureTemplate.java.patch

@@ -18,18 +18,18 @@
  
  public class StructureTemplate {
  
-@@ -73,6 +77,11 @@
-     public final List<StructureTemplate.StructureEntityInfo> entityInfoList = Lists.newArrayList();
+@@ -74,6 +78,11 @@
      private Vec3i size;
      private String author;
-+
+ 
 +    // CraftBukkit start - data containers
 +    private static final CraftPersistentDataTypeRegistry DATA_TYPE_REGISTRY = new CraftPersistentDataTypeRegistry();
 +    public CraftPersistentDataContainer persistentDataContainer = new CraftPersistentDataContainer(StructureTemplate.DATA_TYPE_REGISTRY);
 +    // CraftBukkit end
- 
++
      public StructureTemplate() {
          this.size = Vec3i.ZERO;
+         this.author = "?";
 @@ -147,7 +156,7 @@
      }
  
@@ -141,6 +141,15 @@
      }
  
      private void loadPalette(HolderGetter<Block> blockLookup, ListTag palette, ListTag blocks) {
+@@ -840,7 +889,7 @@
+     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 - Fix CME due to this collection being shared across threads
+         @Nullable
+         private List<StructureTemplate.JigsawBlockInfo> cachedJigsaws;
+ 
 @@ -924,7 +973,7 @@
          public BlockState stateFor(int id) {
              BlockState iblockdata = (BlockState) this.ids.byId(id);