Allow "post processing" of chunks (#658)

* begin allowing "post processing" of chunks
 - restores legacy capability to continue saving edits in the background after sending the chunks
 - speeds up the edit clientside
 - nail in the coffin of the terrible and staticly coded coreedit
 - We should totally make IronGolem work so Core* is no longer used by anyone

* begin allowing background history saving

* Handle post processors in queues properly

* Use futures for postprocessing so we're not waiting for them needlessly

* better use of closed boolean

* Reword

worldedit-core/src/main/java/com/sk89q/worldedit/extent/MaskingExtent.java

@@ -36,6 +36,9 @@ import com.sk89q.worldedit.math.BlockVector3;
 import com.sk89q.worldedit.world.biome.BiomeType;
 import com.sk89q.worldedit.world.block.BlockStateHolder;
 
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.Future;
+
 import static com.google.common.base.Preconditions.checkNotNull;
 
 /**
@@ -106,6 +109,12 @@ public class MaskingExtent extends AbstractDelegateExtent implements IBatchProce
         return filter.filter(chunk, get, set, MaskingExtent.this);
     }
 
+    @Override
+    public Future<IChunkSet> postProcessSet(IChunk chunk, IChunkGet get, IChunkSet set) {
+        // This should not do anything otherwise dangerous...
+        return CompletableFuture.completedFuture(set);
+    }
+
     @Override
     public void applyBlock(final FilterBlock block) {
         if (!this.mask.test(block)) {