Rework async chunk api implementation

Firstly, the old methods all routed to the CompletableFuture method.
However, the CF method could not guarantee that if the caller
was off-main that the future would be "completed" on-main. Since
the callback methods used the CF one, this meant that the callback
methods did not guarantee that the callbacks were to be called on
the main thread.

Now, all methods route to getChunkAtAsync(x, z, gen, urgent, cb)
so that the methods with the callback are guaranteed to invoke
the callback on the main thread. The CF behavior remains unchanged;
it may still appear to complete on main if invoked off-main.

Secondly, remove the scheduleOnMain invocation in the async
chunk completion. This unnecessarily delays the callback
by 1 tick.

Thirdly, add getChunksAtAsync(minX, minZ, maxX, maxZ, ...) which
will load chunks within an area. This method is provided as a helper
as keeping all chunks loaded within an area can be complicated to
implement for plugins (due to the lacking ticket API), and is
already implemented internally anyways.

Fourthly, remove the ticket addition that occured with getChunkAt
and getChunkAtAsync. The ticket addition may delay the unloading
of the chunk unnecessarily. It also fixes a very rare timing bug
where the future/callback would be completed after the chunk
unloads.

patches/server/0634-Add-API-for-item-entity-health.patch

@@ -1,34 +0,0 @@
-From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
-From: Jake Potrebic <jake.m.potrebic@gmail.com>
-Date: Sat, 28 Aug 2021 09:00:45 -0700
-Subject: [PATCH] Add API for item entity health
-
-== AT ==
-public net.minecraft.world.entity.item.ItemEntity health
-
-diff --git a/src/main/java/org/bukkit/craftbukkit/entity/CraftItem.java b/src/main/java/org/bukkit/craftbukkit/entity/CraftItem.java
-index 4a15c3786edbfeae3367c0b20fb6aee11d62aea6..1a291dd8a287db30e71dcb315599fc4b038764c4 100644
---- a/src/main/java/org/bukkit/craftbukkit/entity/CraftItem.java
-+++ b/src/main/java/org/bukkit/craftbukkit/entity/CraftItem.java
-@@ -98,6 +98,21 @@ public class CraftItem extends CraftEntity implements Item {
-      public void setWillAge(boolean willAge) {
-         this.getHandle().age = willAge ? 0 : NO_AGE_TIME;
-      }
-+
-+    @Override
-+    public int getHealth() {
-+        return this.getHandle().health;
-+    }
-+
-+    @Override
-+    public void setHealth(int health) {
-+        if (health <= 0) {
-+            this.getHandle().getItem().onDestroyed(this.getHandle());
-+            this.getHandle().discard(org.bukkit.event.entity.EntityRemoveEvent.Cause.PLUGIN);
-+        } else {
-+            this.getHandle().health = health;
-+        }
-+    }
-     // Paper end
- 
-     @Override