Code review comment for lp:~james-w/aptdaemon/fix-455861

I think we still need some synchronization here because stop() maybe called by the aptdaemon when there are still unprocessed io events. We also need to ensure that all pending io events are processed because otherwise we may kill debconf-communicate (in _hangup) when it has not processed all messages yet, this could let to "forgetting" some of the debconf communication because it never quite made it to the socket.