moved the request scope creation to the ContainerAwareHttpKernel class

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | symfony/symfony-docs#2343

While updating the scope documentation, I realized that the request scope was created in the FrameworkBundle while the HttpKernel that manages it was in the HttpKernel component. So, this PR makes things more consistent.