Endpoints that return many concepts are slow

[stefandesu]

When requesting for example /voc/concepts with a high limit parameter, there is a high "waiting time" caused by slowness in jskos-server (that's on top of network transfer time). I've determined that ~2/3 of that time is due to dynamically adding narrower to concepts (see #61).

Priority is low since this is not a usual use case, but in the long run, I would like to try to switch from MongoDB to PostgreSQL because I expect performance improvements for anything database related.