Fix again FG #11 (mixing daemonize and auto-restart).

Main changes:
* don't modify (clean) the on_exit vector on cleanup, but detach its shared_ptr instead;
* define ActorImpl::undaemonize() for daemon cleanup, and use it instead of using an
  on_exit callback (ensures that it is not added multiple times);
* complete test 'tesh-s4u-actor-autorestart' with an auto-restarting daemon.