Server-pushed events

[JaneJeon]

E.g.

• rather than hitting the database every request, cache id => req.user and make a node instance listen to the postgres changes. When a user change comes through, bust that cache.
• rather than having to hit settings every request (see Advanced access control & revised policies #143), cache settings and basically only bust it when the node is notified of a settings change.

This can probably also help with #158, as having a stream of changes makes it easy to save it.

[JaneJeon]

I'll probably end up relying on https://github.com/zeit/swr and skip the whole "streaming update" thing as live updates aren't important for this application. However, this does mean that the "fetchers" for react-admin needs to be configured to use swr.