Make a copy for worldloadevent to avoid como's - fixes #1453

If you try to load a world inresponse to another world loading,
a concurrent modification exception will throw.

This avoids that by making a defensive copy of the list for the server init stage.

you might  want to do this if you want to guarantee your world loads
immediately after the vanilla worlds before another plugin has a chance
to load worlds during POST_WORLD plugin stage.

Spigot-Server-Patches/0375-Optimize-Server-World-Map.patch

@@ -1,4 +1,4 @@
-From a986a6ac3b035444524be7214677b83c7094fc5d Mon Sep 17 00:00:00 2001
+From c0d6698eb4af5eb4e477740679c2ebcf0b5cd5e0 Mon Sep 17 00:00:00 2001
 From: Aikar <aikar@aikar.co>
 Date: Mon, 17 Sep 2018 23:37:31 -0400
 Subject: [PATCH] Optimize Server World Map
@@ -217,7 +217,7 @@ index 0000000000..af9e4455c6
 +    }
 +}
 diff --git a/src/main/java/net/minecraft/server/MinecraftServer.java b/src/main/java/net/minecraft/server/MinecraftServer.java
-index fb62320310..2912e9ec68 100644
+index fb62320310..98d182fdb8 100644
 --- a/src/main/java/net/minecraft/server/MinecraftServer.java
 +++ b/src/main/java/net/minecraft/server/MinecraftServer.java
 @@ -87,7 +87,7 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
@@ -229,6 +229,15 @@ index fb62320310..2912e9ec68 100644
      private PlayerList s;
      private boolean isRunning = true;
      private boolean isRestarting = false; // Paper - flag to signify we're attempting to restart
+@@ -550,7 +550,7 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
+             }
+         }
+ 
+-        for (WorldServer world : this.getWorlds()) {
++        for (WorldServer world : com.google.common.collect.Lists.newArrayList(this.getWorlds())) { // Paper - avoid como if 1 world triggers another world
+             this.server.getPluginManager().callEvent(new org.bukkit.event.world.WorldLoadEvent(world.getWorld()));
+         }
+         // CraftBukkit end
 -- 
 2.19.0