Change forEach() to forEachSeries()

The loadObject() has been modified from parallel to serial. Because when the second collection of needs when the first collection is loaded in parallel will result in an error.

```javascript
SecondModel.schema.path('ref').validate(function(value, respond) {
    FirstModel.findById(value, function(err, doc) {
        if (err || !doc) return respond(false)
        respond(true)
    })
}, 'validate fail')
```