also ensure we never process already valid entities. this shouldnt be possible as of recent
commits as we made the entity slice array safer, but doesn't hurt for this logic to be safe too
incase that patch got dropped in a future version by accident/necessarily
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
Aikar