Numerous fixes to entity related changes

While it wasn't really "broken" before, if plugins use NMS
(which they really should't be) and mess with entity management
themselves, and get it wrong, they could ultimately corrupt our
state expectations.

I've been unable to reproduce any issues locally, but these changes
are the result of me analyzing the code pretty deeply and seeing
about how to make it more durable to abnormal usage.

Any servers seeing oddities, please run with -Ddebug.entities=true
and send me any logs triggered.

Spigot-Server-Patches/Always-process-chunk-removal-in-removeEntity.patch

@@ -8,7 +8,7 @@ which can keep them in the chunk when they shouldnt be if done
 during entity ticking.
 
 diff --git a/src/main/java/net/minecraft/server/World.java b/src/main/java/net/minecraft/server/World.java
-index 08596bf3c..f93adc860 100644
+index 1929e9d2ae..1cbe6e17b7 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 IEntityAccess, GeneratorAccess, IIBlockAc
@@ -20,9 +20,8 @@ index 08596bf3c..f93adc860 100644
          int i = entity.chunkX;
          int j = entity.chunkZ;
  
-@@ -0,0 +0,0 @@ public abstract class World implements IEntityAccess, GeneratorAccess, IIBlockAc
-             this.getChunkAt(i, j).b(entity);
-         }
+         Chunk chunk = entity.getCurrentChunk(); // Paper
+         if (chunk != null) chunk.removeEntity(entity); // Paper
  
 +        if (!guardEntityList) { // Spigot - It will get removed after the tick if we are ticking // Paper - always remove from current chunk above
          // CraftBukkit start - Decrement loop variable field if we've already ticked this entity