docs: function saveOldSessions() {
      var db = self.store;
      var oldDestroy = db.destroy;
      db.destroy = newDestroy;
      return;
  
      // rename instead of delete
      function newDestroy(sid, cb) {
        db.get(sid, function(err, session) {
          if (err) return cb(err);
          if (!session) return cb();
          db.set(sid + '_d', session, function(err) {
            if (err) return cb(err);
            oldDestroy.call(db, sid, cb);
          });
        });
      }
    }