Mark chunk as active if ChunkUnloadEvent is cancelled

Potentially related to GH-169

Spigot-Server-Patches/Optimize-Chunk-Unload-Queue.patch

@@ -156,6 +156,17 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
                              if (neighbor != null) {
                                  neighbor.setNeighborUnloaded(-x, -z);
                                  chunk.setNeighborUnloaded(x, z);
+                             }
+                         }
+                     }
++                // Paper start
++                } else {
++                    // Unload event is cancelled, make sure this chunk is marked active
++                    markChunkActive(chunk);
++                // Paper end
+                 }
+             }
+             // CraftBukkit end
 @@ -0,0 +0,0 @@ public class ChunkProviderServer implements IChunkProvider {
      public boolean e(int i, int j) {
          return this.chunks.containsKey(LongHash.toLong(i, j)); // CraftBukkit