Use java 8's java.time

[RickMoynihan]

We should update to use the latest java 8 time API, and move away from using Date/#inst as a means of coercing between RDF time classes.

https://lambdaisland.com/blog/26-07-2017-dates-in-clojure-making-sense-of-the-mess

Also Juxt's tick API seems to be even better than clj-time, as it's built on the underlying java.time so we should consider adopting that more widely.