Do preloads which are at the same levels in parallel

[yansal]

Let's say we have the following models

type Foo struct {
    Bar *Bar
    Bazs []Baz
}
type Bar struct {}
type Baz struct {}

The Bar and Bazs fields can be preloaded in parallel.

[novln]

If memory served, @thoas asked to remove this feature because it could stress the database connection: that's why it's sequential now.

You can find the previous implementation at this commit: 1d47b45#diff-954de6d7c840669472378e118a132334L241