unmark chunk as unloading when unload is cancelled

no obviousy bugs caused by this at the moment, but we may need to clean up process to be like
how I use to have it before vanilla did it, and we shouldn't leave this boolean in an invalid state.

Spigot-Server-Patches/0153-Unmark-chunk-as-unloading-when-unload-is-cancelled.patch

@@ -0,0 +1,34 @@
+From d2cbd64700e4bd54a1aec471f69977b63c6ea2ec Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Thu, 12 May 2016 02:03:56 -0400
+Subject: [PATCH] Unmark chunk as unloading when unload is cancelled
+
+
+diff --git a/src/main/java/net/minecraft/server/Chunk.java b/src/main/java/net/minecraft/server/Chunk.java
+index 4953e97..1981ae4 100644
+--- a/src/main/java/net/minecraft/server/Chunk.java
++++ b/src/main/java/net/minecraft/server/Chunk.java
+@@ -46,7 +46,7 @@ public class Chunk {
+     private long w;
+     private int x;
+     private ConcurrentLinkedQueue<BlockPosition> y;
+-    public boolean d;
++    public boolean d;public void setShouldUnload(boolean unload) { this.d = unload; } // Paper // OBFHELPER
+     protected gnu.trove.map.hash.TObjectIntHashMap<Class> entityCount = new gnu.trove.map.hash.TObjectIntHashMap<Class>(); // Spigot
+     public int lightUpdates; // Paper - Number of queued light updates for this chunk
+ 
+diff --git a/src/main/java/net/minecraft/server/ChunkProviderServer.java b/src/main/java/net/minecraft/server/ChunkProviderServer.java
+index 694f3d5..f67dbae 100644
+--- a/src/main/java/net/minecraft/server/ChunkProviderServer.java
++++ b/src/main/java/net/minecraft/server/ChunkProviderServer.java
+@@ -309,6 +309,7 @@ public class ChunkProviderServer implements IChunkProvider {
+                     Chunk chunk = (Chunk) this.chunks.get(olong);
+ 
+                     if (chunk != null && chunk.d) {
++                        chunk.setShouldUnload(false); // Paper
+                         if (chunk.hasLightUpdates()) continue; // Paper - Don't unload chunks with pending light updates.
+                         // CraftBukkit start
+                         ChunkUnloadEvent event = new ChunkUnloadEvent(chunk.bukkitChunk);
+-- 
+2.8.2
+