docs: function del(keys, cb){
    if(!keys || keys.length<=0){
      if(cb) return cb(new Error('empty key'));
      else throw new Error('empty key');
    }
    var fns = [];
    for(var j in this._stores){
      var store = this._stores[j];
      fns.push(store.del.bind(store, keys));
    }
    if(cb){
      concurrent(fns, cb);
    }else{
      var fiber = fibext(), rbw = false;
      concurrent(fns, function(){
        fiber.resume.apply(fiber, arguments);
        rbw = true;
      });
      rbw || fiber.wait();
    }
  }