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/Provide-Chunk-Coordinates-as-a-Long-API.patch

@@ -7,7 +7,7 @@ Allows you to easily access the chunks X/z as a long, and a method
 to look up by the long key too.
 
 diff --git a/src/main/java/org/bukkit/Chunk.java b/src/main/java/org/bukkit/Chunk.java
-index 079b9feb..c75bce07 100644
+index 079b9febe..b347a3ccf 100644
 --- a/src/main/java/org/bukkit/Chunk.java
 +++ b/src/main/java/org/bukkit/Chunk.java
 @@ -0,0 +0,0 @@ public interface Chunk {
@@ -19,7 +19,24 @@ index 079b9feb..c75bce07 100644
 +     * @return The Chunks X and Z coordinates packed into a long
 +     */
 +    default long getChunkKey() {
-+        return (long) getX() & 0xffffffffL | ((long) getZ() & 0xffffffffL) << 32;
++        return getChunkKey(getX(), getZ());
++    }
++
++    /**
++     * @param loc Location to get chunk key
++     * @return Location's chunk coordinates packed into a long
++     */
++    static long getChunkKey(Location loc) {
++        return getChunkKey((int) Math.floor(loc.getX()) << 4, (int) Math.floor(loc.getZ()) << 4);
++    }
++
++    /**
++     * @param x X Coordinate
++     * @param z Z Coordinate
++     * @return Chunk coordinates packed into a long
++     */
++    static long getChunkKey(int x, int z) {
++        return (long) x & 0xffffffffL | ((long) z & 0xffffffffL) << 32;
 +    }
 +    // Paper end
 +
@@ -27,7 +44,7 @@ index 079b9feb..c75bce07 100644
       * Gets the world containing this chunk
       *
 diff --git a/src/main/java/org/bukkit/World.java b/src/main/java/org/bukkit/World.java
-index dbbcfec9..724088ec 100644
+index 5e6cb56ab..e451ca611 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 {
@@ -50,6 +67,6 @@ index dbbcfec9..724088ec 100644
 +    // Paper end
 +
      /**
-      * Used by {@link World#getChunkAtAsync(Location,ChunkLoadCallback)} methods
-      * to request a {@link Chunk} to be loaded, with this callback receiving
+      * Checks if the specified {@link Chunk} is loaded
+      *
 --