Nginx front-end proxy does not check that back-end omero-web is alive

[manics]

Haproxy connects to Nginx on idr-omeroreadonly-N which is always running even if OMERO.web is unavailable (e.g. during a restart). We should figure out a proper liveness check e.g. check the HTTP status code from Nginx

[manics]

Correction: It's a front-end nginx and not haproxy that connects to the backends. The open-source Nginx does not seem to support HTTP status-based checks: https://docs.nginx.com/nginx/admin-guide/load-balancer/http-health-check/

I think we should replace it with haproxy.

[manics]

An addition benefit is we could use cookie based session pinning instead of by IP/24 (another limitation of the open-source Nginx)