Defensively copy chunk data in upgradeChunkTag

The data provided is always from the regionfile thread, which
does not copy the data out. So if two separate calls need
the data, then there's going to be a problem.

patches/server/Asynchronous-chunk-IO-and-loading.patch

@@ -3219,6 +3219,14 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
              CompoundTag level = nbt.getCompound("Level");
              if (level.getBoolean("TerrainPopulated")) {
 @@ -0,0 +0,0 @@ public class ChunkStorage implements AutoCloseable {
+ 
+     public CompoundTag upgradeChunkTag(ResourceKey<LevelStem> resourcekey, Supplier<DimensionDataStorage> supplier, CompoundTag nbttagcompound, Optional<ResourceKey<Codec<? extends ChunkGenerator>>> optional, ChunkPos pos, @Nullable LevelAccessor generatoraccess) throws IOException {
+         // CraftBukkit end
++        nbttagcompound = nbttagcompound.copy(); // Paper - defensive copy, another thread might modify this
+         int i = ChunkStorage.getVersion(nbttagcompound);
+ 
+         // CraftBukkit start
+@@ -0,0 +0,0 @@ public class ChunkStorage implements AutoCloseable {
          if (i < 1493) {
              nbttagcompound = NbtUtils.update(this.fixerUpper, DataFixTypes.CHUNK, nbttagcompound, i, 1493);
              if (nbttagcompound.getCompound("Level").getBoolean("hasLegacyStructureData")) {