Fix #639

looks like we too speedy

worldedit-core/src/main/java/com/boydti/fawe/beta/implementation/queue/SingleThreadQueueExtent.java

@@ -216,7 +216,7 @@ public class SingleThreadQueueExtent extends ExtentBatchProcessorHolder implemen
     }
 
     @Override
-    public final IQueueChunk getOrCreateChunk(int x, int z) {
+    public synchronized final IQueueChunk getOrCreateChunk(int x, int z) {
         final long pair = (long) x << 32 | z & 0xffffffffL;
         if (pair == lastPair) {
             return lastChunk;