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/Fix-Sending-Chunks-to-Client.patch

@@ -14,7 +14,7 @@ This fix always sends chunks to the client, and simply updates
 the client anytime post processing is triggered with the new chunk data.
 
 diff --git a/src/main/java/net/minecraft/server/Chunk.java b/src/main/java/net/minecraft/server/Chunk.java
-index d848a97bd..2bcbf8902 100644
+index 0d51c1baeb..46804203fe 100644
 --- a/src/main/java/net/minecraft/server/Chunk.java
 +++ b/src/main/java/net/minecraft/server/Chunk.java
 @@ -0,0 +0,0 @@ public class Chunk implements IChunkAccess {
@@ -41,7 +41,7 @@ index d848a97bd..2bcbf8902 100644
      }
  
 diff --git a/src/main/java/net/minecraft/server/PlayerChunk.java b/src/main/java/net/minecraft/server/PlayerChunk.java
-index e4cf8548d..ac5d15809 100644
+index e4cf8548d3..ac5d158093 100644
 --- a/src/main/java/net/minecraft/server/PlayerChunk.java
 +++ b/src/main/java/net/minecraft/server/PlayerChunk.java
 @@ -0,0 +0,0 @@ public class PlayerChunk {