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/Optimize-Server-World-Map.patch

@@ -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
 @@ -0,0 +0,0 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
@@ -229,4 +229,13 @@ index fb62320310..2912e9ec68 100644
      private PlayerList s;
      private boolean isRunning = true;
      private boolean isRestarting = false; // Paper - flag to signify we're attempting to restart
+@@ -0,0 +0,0 @@ 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
 --