Async always exits child asyncs with 'shutdown', which prevents some async processes receiving the intended exit reason

This issue was originally noticed causing MB-54050 which is being fixed in Trinity. However, the underlying issue is that when terminating children of asyncs we replace the exit reason with shutdown, meaning that children that handle specific exit reasons differently will potentially be behaving incorrectly. This behaviour was introduced here: https://review.couchbase.org/c/ns_server/+/104297