Code review comment for lp://qastaging/~afrantzis/mir/fix-1441620-simple-dispatch-thread-self-destruction-test

Another option would be to abort instead of detaching the thread; that's implemented here: https://code.launchpad.net/~raof/mir/remove-eventloop-self-immolation/+merge/255767

Rather than passing in a reference to a promise, would it make more sense to have a method
std::shared_future<void> shutdown_notifier()
and have ~SimpleDispatchThread set_value_at_thread_exit() in the case of destruction-from-eventloop-thread?

I'm happy with either.