Fixed world.getHighestBlockYAt(x,z) returning 0 for existing but not loaded chunks. This fixes BUKKIT-327. Thanks for dredhorse for the pull request!

By: Nathan Adams <dinnerbone@dinnerbone.com>

paper-server/src/main/java/org/bukkit/craftbukkit/CraftWorld.java

@@ -74,6 +74,10 @@ public class CraftWorld implements World {
     }
 
     public int getHighestBlockYAt(int x, int z) {
+        if (!isChunkLoaded(x >> 4, z >> 4)){
+            loadChunk(x >> 4, z >> 4);
+        }
+
         return world.getHighestBlockYAt(x, z);
     }