docs: function set(kvs, options, cb){
    if(Object.keys(kvs).length<=0){
      if(cb) return cb(new Error('empty key and value'));
      throw new Error('empty key and value');
    }
  
    var fns = [];
    for(var s in this._stores){
      var store = this._stores[s];
      fns.push(store.set.bind(store, kvs, options));
    }
    if(cb){
      concurrent(fns, cb);
    }else{
      var fiber = fibext(), rbw = false;//resume before wait
      concurrent(fns, function(){
        fiber.resume.apply(fiber, arguments);
        rbw = true;
      });
      rbw || fiber.wait();
    }  
  }