[Bug Report] MessageBox only shows the last one when calling it multi times synchronously

### Element UI version
2.13.2

### OS/Browsers version
all

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/pen/

### Steps to reproduce
1. click "show msg" button
2. click "show 2 msgs" button
3. click "show 2 msgs async" button

### What is Expected?
1. 1 MessageBox appears
2. 2 MessageBox appear in the function's calling order
3. 2 MessageBox appear in the function's calling order

### What is actually happening?
1. 1 MessageBox appears
2. only the last MessageBox appears
3. 2 MessageBox appear in the function's calling order

As the reproducting codepen shows, when calling `this.$msgbox` multi time synchronously by clicking "show 2 msgs" button, it only shows the last one.
While when calling it synchronously with `setTimeout`, all the MessageBox will show in order.