message: Add cache for Vlist
  Add functions and members used to add cache for Vlist
diff: @@ -151,6 +151,19 @@ private:
  void load_obj2values(int _mode);
  void load_pre2values(int _mode);
  + // functions used to build cache
  + void load_cache();
  + void get_important_preID();
  + std::vector <TYPE_PREDICATE_ID> important_preID;
  + void load_important_sub2values();
  + void load_important_obj2values();
  + void build_CacheOfSub2values();
  + void build_CacheOfObj2values();
  + void get_important_subID();
  + void get_important_objID();
  + std::priority_queue <KEY_SIZE_VALUE> important_subID;
  + std::priority_queue <KEY_SIZE_VALUE> important_objID;
  +
  //triple num per group for insert/delete
  //can not be too high, otherwise the heap will over
  static const int GROUP_SIZE = 1000;
  