Don't update entity trackers for worlds without players

PlayerList#moveToWorld already untracks the player from the player list,
meaning that we do not need to worry about this untracking players
who've left the world, The server also untracks a player during
disconnect, handing yet another case.

If we don't need to untrack players who've left the world, it should be
reasonably save to do this, as we're not going to be performing any
server->client updates here, which is what this code is intended to do,
and all players should be untracked.

Spigot-Server-Patches/0428-Don-t-update-entity-trackers-for-worlds-without-play.patch

@@ -0,0 +1,31 @@
+From 1b01715133963b2fda50c547261b419386e12cc3 Mon Sep 17 00:00:00 2001
+From: Shane Freeder <theboyetronic@gmail.com>
+Date: Sun, 24 Mar 2019 18:52:31 +0000
+Subject: [PATCH] Don't update entity trackers for worlds without players
+
+PlayerList#moveToWorld already untracks the player from the player list,
+meaning that we do not need to worry about this untracking players
+who've left the world, The server also untracks a player during
+disconnect, handing yet another case.
+
+If we don't need to untrack players who've left the world, it should be
+reasonably save to do this, as we're not going to be performing any
+server->client updates here, which is what this code is intended to do,
+and all players should be untracked.
+
+diff --git a/src/main/java/net/minecraft/server/MinecraftServer.java b/src/main/java/net/minecraft/server/MinecraftServer.java
+index ce39ea09e8..b93fccf919 100644
+--- a/src/main/java/net/minecraft/server/MinecraftServer.java
++++ b/src/main/java/net/minecraft/server/MinecraftServer.java
+@@ -1113,7 +1113,7 @@ public abstract class MinecraftServer implements IAsyncTaskHandler, IMojangStati
+ 
+                 this.methodProfiler.exit();
+                 this.methodProfiler.enter("tracker");
+-                worldserver.getTracker().updatePlayers();
++                if (playerList.players.size() > 0) worldserver.getTracker().updatePlayers(); // Paper - No players, why spend time tracking them? (See patch)
+                 this.methodProfiler.exit();
+                 this.methodProfiler.exit();
+                 worldserver.explosionDensityCache.clear(); // Paper - Optimize explosions
+-- 
+2.21.0
+