Saving related properties of Models

The easiest way to describe this feature is by providing a syntactical example:

``` js
const company = new Company({
email: 'famous@company.com',
employees: [
new Employee({ name: 'John Doe' }), // New Employee
new Employee({ id: 6 }, false), // Existing Employee
],
});

await company.save();
```

The code above should insert a new Company to the database, but before that, the new Employee (first in the array) shall be stored. Related object trees should be stored recursively, in an order defined by the relations of the referenced objects.

Start with the provided Company/Employee example and trace the existing model persistence entry point used by company.save(). Done means recursively storing new related objects before their parent, preserving existing related records, and respecting relation order; verify these cases with focused tests.

Written by the indexing model from the issue text.