[Major] Ensures world tick safety, #4 #5 #6

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

@@ -139,8 +139,9 @@ public class MixinMinecraftServer {
         }
         Akari.silentTiming = true; // Disable timings
         Akari.STAGE_TICK.submit(() -> {
-            for (int i = 0; i < worlds.size(); ++i) {
+            // Never tick one world concurrently!
-                WorldServer world = worlds.get(i);
+            for (int i = 1; i <= worlds.size(); ++i) {
+                WorldServer world = worlds.get(i < worlds.size() ? i : 0);
                 tickEntities(world);
             }
         }, null);