subprocess_middleware:More robust _suspend_recursive

We've seen in production environment that NoSuchProcess seems to be thrown from proc.children(recursive=True). It's unclear if proc is actually gone, despite being suspended earlier in this function, or if the error comes from some child disappearing in a race during listing of children.

We can determine this however, using proc.is_running(). If it returns true, our target process is already reaped, and there is nothing more to do.

Otherwise, we assume the error to come from a child disappearing during listing, and try again.