Entity add to world fixes - #1223

1) Chunk Registration might kill an entity, don't add it to the world if it did!

2) By default, entities are added to the world per slice iteration.
This opens risk of the slices being manipulated during chunk add if an
EntityAddToWorldEvent spawns an entity into this chunk.
Fix this by differing entity add to world for all entities at the same time

3) If a duplicate entity is attempted to add to the world of an entity, and
the original entity is dead, overwrite it as the logic does for unloaod queued entities.

Should hopefully finish up issues with #1223

Spigot-Server-Patches/Prevent-Saving-Bad-entities-to-chunks.patch

@@ -56,30 +56,4 @@ index bcce5e8b7e..bad287fca4 100644
  
          nbttagcompound.set("Entities", nbttaglist1);
          NBTTagList nbttaglist2 = new NBTTagList();
-diff --git a/src/main/java/net/minecraft/server/World.java b/src/main/java/net/minecraft/server/World.java
-index c53e77b821..2a4405638e 100644
---- a/src/main/java/net/minecraft/server/World.java
-+++ b/src/main/java/net/minecraft/server/World.java
-@@ -0,0 +0,0 @@ public abstract class World implements IBlockAccess {
-             }
- 
-             this.getChunkAt(i, j).a(entity);
--            this.entityList.add(entity);
-+            if (!entity.dead) this.entityList.add(entity); // Paper - don't add dead entities, chunk registration may of killed it
-             this.b(entity);
-             return true;
-         }
-diff --git a/src/main/java/net/minecraft/server/WorldServer.java b/src/main/java/net/minecraft/server/WorldServer.java
-index 1244baf45a..1763d9f94d 100644
---- a/src/main/java/net/minecraft/server/WorldServer.java
-+++ b/src/main/java/net/minecraft/server/WorldServer.java
-@@ -0,0 +0,0 @@ public class WorldServer extends World implements IAsyncTaskHandler {
-             if (this.entitiesByUUID.containsKey(uuid)) {
-                 Entity entity1 = (Entity) this.entitiesByUUID.get(uuid);
- 
--                if (this.f.contains(entity1)) {
-+                if (this.f.contains(entity1) || entity1.dead) { // Paper - overwrite the current dead one
-                     this.f.remove(entity1);
-                 } else {
-                     if (!(entity instanceof EntityHuman)) {
 --