[Contrib blocker] SubRequestBuffer service went missing in 4.x

In the transition from 3.x to 4.x the `SubRequestBuffer` service was removed but we never got round to restoring it. The service was removed as part of the work in #812 which was one of the last PRs that was blocking the first 4.x alpha release.

It was probably just forgotten in the rush to create the alpha. Evidence for this is that we still have the service definition in `graphql.services.yml`, but the service class is missing.

This is a major (or even critical) issue since this is blocking migrations from GraphQL 3.x to 4.x. Also the [GraphQL Core Schema](https://www.drupal.org/project/graphql_core_schema) contrib module depends on this service and cannot move out of beta until this is restored.