cubicweb #2449207 Provide lightweigth up status page [open]

It would be usefull for monitoring purposes (and varnish integration) to have a URL such as /status/up which would just return a simple HTTP 200 "OK" page. This would require no security no database access etc. and take a very small memory footprint so it can be queried intensivelly.

done in<not specified>
closed by<not specified>