Rework Async Chunks API in prep for merge, add utility

This adds a new Future based, Consumer<Chunk> based, and ability
to control whether or not to generate to the Async Chunk API.

Until Async Chunks merges, these API's are still synchronous, but
this commit will allow plugins to start using the API's in use
with the Async Chunks beta.

Spigot-API-Patches/isChunkGenerated-API.patch

@@ -3,10 +3,9 @@ From: cswhite2000 <18whitechristop@gmail.com>
 Date: Tue, 21 Aug 2018 19:39:46 -0700
 Subject: [PATCH] isChunkGenerated API
 
-Resolves #1329
 
 diff --git a/src/main/java/org/bukkit/Location.java b/src/main/java/org/bukkit/Location.java
-index 7e1ee875..9457832b 100644
+index 7e1ee875e..9457832bc 100644
 --- a/src/main/java/org/bukkit/Location.java
 +++ b/src/main/java/org/bukkit/Location.java
 @@ -0,0 +0,0 @@ import org.bukkit.util.NumberConversions;
@@ -34,14 +33,13 @@ index 7e1ee875..9457832b 100644
      /**
       * Sets the position of this Location and returns itself
 diff --git a/src/main/java/org/bukkit/World.java b/src/main/java/org/bukkit/World.java
-index d4bfbad3..53764fae 100644
+index 348f1c98d..9c06fc163 100644
 --- a/src/main/java/org/bukkit/World.java
 +++ b/src/main/java/org/bukkit/World.java
 @@ -0,0 +0,0 @@ public interface World extends PluginMessageRecipient, Metadatable {
-     public default Chunk getChunkAt(long chunkKey) {
          return getChunkAt((int) chunkKey, (int) (chunkKey >> 32));
      }
-+
+ 
 +    /**
 +     * Checks if a {@link Chunk} has been generated at the specified chunk key,
 +     * which is the X and Z packed into a long.
@@ -61,7 +59,8 @@ index d4bfbad3..53764fae 100644
 +     * @return true if the chunk has been generated, otherwise false
 +     */
 +    public boolean isChunkGenerated(int x, int z);
-     // Paper end
- 
++
      /**
+      * This is the Legacy API before Java 8 was supported. Java 8 Consumer is provided,
+      * as well as future support
 --