## Description
This PR fixes a bug where the engine does not actually execute root
fields in parallel when it is supposed to be doing so.

Consider the following GraphQL query that is invoking a typescript
function that sleeps for 5 seconds three times:

```graphql
query MyQuery {
  test1: app_hello(name: "test1")
  test2: app_hello(name: "test2")
  test3: app_hello(name: "test3")
}
```

This should execute in 5 seconds, as these should be run in parallel.
Instead, it actually takes 15 seconds as they are run sequentially.

Here's a trace from before the change:

![image](https://github.com/hasura/v3-engine/assets/1214352/d52d99e7-b4da-4bbb-bbf9-5155ef568d76)

And here's a trace from after the change:

![image](https://github.com/hasura/v3-engine/assets/1214352/26a3ac1f-9e9c-4067-aa1b-62aaa4b293c2)

## Changelog

### Product

_(Select all products this will be available in)_

- [x] community-edition
- [x] cloud
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### Type

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_(Select only one. In case of multiple, choose the most appropriate)_

- [ ] highlight
- [ ] enhancement
- [x] bugfix
- [ ] behaviour-change
- [ ] performance-enhancement
- [ ] security-fix
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### Changelog entry

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Fixed a bug in the engine that prevented the parallel execution of query
root fields

<!-- changelog-entry : end : DO NOT REMOVE -->

<!-- changelog : end : DO NOT REMOVE -->

V3_GIT_ORIGIN_REV_ID: 2d4dd195037d49577608892fc6fa0237d8fd26c4