Implement systemd notify

[JanKoppe]

https://pypi.python.org/pypi/sdnotify

I would like to see pyCA communicate with systemd via notifications, so that we can leverage the systemd watchdog. Sometimes, some processes will hang themselves, which is unacceptable in a production environment.

If pyCA would use sdnotify, we could change the service type to notify and set a Watchdog that is about twice of the default check intervall and send a notification on every control loop iteration. A stuck worker would then be forcefully restarted by systemd, ensuring that the basic functionionality will work.

Of course this is much harder to do on the capture worker - I would leave this out for now.