Restore World.loadChunkAsync API - but load chunks sync

We are still missing Async Chunk Loading, but plugins may be
depending on this API, so it missing blocks upgrading.

Spigot-Server-Patches/Add-async-chunk-load-API.patch

@@ -0,0 +1,39 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Sat, 21 Jul 2018 16:55:04 -0400
+Subject: [PATCH] Add async chunk load API
+
+
+diff --git a/src/main/java/org/bukkit/craftbukkit/CraftWorld.java b/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
+index 0f4a894eb..995e02f1d 100644
+--- a/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
++++ b/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
+@@ -0,0 +0,0 @@ public class CraftWorld implements World {
+         }
+     }
+ 
++    // Paper start - Async chunk load API
++    public void getChunkAtAsync(final int x, final int z, final ChunkLoadCallback callback) {
++        final ChunkProviderServer cps = this.world.getChunkProviderServer();
++        callback.onLoad(cps.getChunkAt(x, z).bukkitChunk); // TODO: Add back async variant
++        /*cps.getChunkAt(x, z, new Runnable() {
++            @Override
++            public void run() {
++                callback.onLoad(cps.getChunkAt(x, z).bukkitChunk);
++            }
++        });*/
++    }
++
++    public void getChunkAtAsync(Block block, ChunkLoadCallback callback) {
++        getChunkAtAsync(block.getX() >> 4, block.getZ() >> 4, callback);
++    }
++
++    public void getChunkAtAsync(Location location, ChunkLoadCallback callback) {
++        getChunkAtAsync(location.getBlockX() >> 4, location.getBlockZ() >> 4, callback);
++    }
++    // Paper end
++
+     public Chunk getChunkAt(int x, int z) {
+         return this.world.getChunkProviderServer().getChunkAt(x, z).bukkitChunk;
+     }
+--