added an optimization option to HandleSet mass data storage structure

source/net/yacy/cora/storage/HandleSet.java

@@ -36,6 +36,7 @@ public interface HandleSet extends Iterable<byte[]>, Cloneable, Serializable {
 
     public byte[] export();
 
+    public void optimize();
 
     /**
      * write a dump of the set to a file. All entries are written in order

source/net/yacy/kelondro/index/RowHandleSet.java

@@ -89,6 +89,11 @@ public final class RowHandleSet implements HandleSet, Iterable<byte[]>, Cloneabl
         return this.index.exportCollection();
     }
 
+    @Override
+    public void optimize() {
+        this.index.sort();
+    }
+    
     /**
      * initialize a HandleSet with the content of a dump
      * @param keylength