Improve rendering performance of nodes and relations API endpoints

[esebastian]

The application holds up pretty well under severe load, but there's some clear bottleneck (firing up the CPU usage) when rendering the JSON responses for the nodes and relations API endpoints:

It is not clear at the moment if the rendering process is requested more times than necessary, but given that when a visualization is published is uncommon for its contents to change a quick win would be to review our cache policy: at the moment we're not caching these responses.