message: Add Obj::set_null, ConstObj::is_null with named column
diff: @@ -116,6 +116,10 @@ public:
  size_t get_link_count(ColKey col_key) const;
  bool is_null(ColKey col_key) const;
  + bool is_null(StringData col_name) const
  + {
  + return is_null(get_column_key(col_name));
  + }
  bool has_backlinks(bool only_strong_links) const;
  size_t get_backlink_count(bool only_strong_links = false) const;
  size_t get_backlink_count(const Table& origin, ColKey origin_col_key) const;
  @@ -198,6 +202,10 @@ public:
  Obj& set(ColKey col_key, util::Optional<U> value, bool is_default = false);
  Obj& set_null(ColKey col_key, bool is_default = false);
  + Obj& set_null(StringData col_name, bool is_default = false)
  + {
  + return set_null(get_column_key(col_name), is_default);
  + }
  Obj& add_int(ColKey col_key, int64_t value);
  Obj& add_int(StringData col_name, int64_t value)
  