Fix plugins calling getChunkAtAsync asynchronously

While this method has async in it's name, it's not actually meant
to be called asynchronously.... It just means IT will load the chunk
asynchronously without blocking main.

So fix this so that if a plugin calls it async, it forces the request back to main thread.

Spigot-Server-Patches/Asynchronous-chunk-IO-and-loading.patch

@@ -4182,6 +4182,18 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +            if (immediate != null) {
 +                return CompletableFuture.completedFuture(immediate.getBukkitChunk());
 +            }
++        } else {
++            CompletableFuture<Chunk> future = new CompletableFuture<Chunk>();
++            world.getMinecraftServer().execute(() -> {
++                getChunkAtAsync(x, z, gen, urgent).whenComplete((chunk, err) -> {
++                    if (err != null) {
++                        future.completeExceptionally(err);
++                    } else {
++                        future.complete(chunk);
++                    }
++                });
++            });
++            return future;
 +        }
 +
 +        return this.world.getChunkProvider().getChunkAtAsynchronously(x, z, gen, urgent).thenComposeAsync((either) -> {

Spigot-Server-Patches/Implement-Chunk-Priority-Urgency-System-for-Chunks.patch

@@ -823,7 +823,7 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 --- 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 {
-             }
+             return future;
          }
  
 +        if (!urgent) {