Add World.getEntity(UUID) API

This is the best way to get an entity when the world and its UUID are known.
It is faster than Server.getEntity(UUID) because it does not have to iterate all worlds

Spigot-Server-Patches/Implement-World.getEntity-UUID-API.patch

@@ -0,0 +1,26 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Brokkonaut <hannos17@gmx.de>
+Date: Tue, 3 Jul 2018 16:08:14 +0200
+Subject: [PATCH] Implement World.getEntity(UUID) API
+
+
+diff --git a/src/main/java/org/bukkit/craftbukkit/CraftWorld.java b/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
+index e2da30d88..210e3bc4e 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 {
+         return list;
+     }
+ 
++    // Paper start - getEntity by UUID API
++    public Entity getEntity(UUID uuid) {
++        Validate.notNull(uuid, "UUID cannot be null");
++        net.minecraft.server.Entity entity = world.getEntity(uuid);
++        return entity == null ? null : entity.getBukkitEntity();
++    }
++    // Paper end
++
+     public void save() {
+     // Spigot start
+         save(true);
+--