In backend, unify timestamp data types

[fm3]

I think I’ve seen java.time.Instant, java.time.LocalDateTime, Long, java.sql.Timestamp and org.joda.time.DateTime for timestamps. We should decide which one works best and stick to it (possibly adding json writes/reads and postgres de/serialization). Looks like duration already pretty consistently uses scala.concurrent.duration. Edit: ok now I also saw java.time.Duration…