[Major] Ensures no concurrent tick one world

sources/src/main/java/io/akarin/server/core/AkarinSlackScheduler.java

@@ -1,9 +1,6 @@
 package io.akarin.server.core;
 
-import org.bukkit.entity.Player;
-
 import io.akarin.api.Akari;
-import net.minecraft.server.ChatMessage;
 import net.minecraft.server.EntityPlayer;
 import net.minecraft.server.MinecraftServer;
 import net.minecraft.server.PacketPlayOutKeepAlive;

sources/src/main/java/io/akarin/server/mixin/core/MixinMinecraftServer.java

@@ -140,15 +140,20 @@ public class MixinMinecraftServer {
         Akari.silentTiming = true; // Disable timings
         Akari.STAGE_TICK.submit(() -> {
             // Never tick one world concurrently!
+            // TODO better treat world index
             for (int i = 1; i <= worlds.size(); ++i) {
                 WorldServer world = worlds.get(i < worlds.size() ? i : 0);
-                tickEntities(world);
+                synchronized (world.tickLock) {
+                    tickEntities(world);
+                }
             }
         }, null);
         
         for (int i = 0; i < worlds.size(); ++i) {
             WorldServer world = worlds.get(i);
-            tickWorld(world);
+            synchronized (world.tickLock) {
+                tickWorld(world);
+            }
         }
         
         Akari.STAGE_TICK.take();