Overhaul and cleanup datetime handling to express it as much as possible in java.time classes

[mrotteveel]

It seems JulianFields.MODIFIED_JULIAN_DAY produces the same values as our custom logic, so it may make sense to replace it. However, it might not be a drop in change.

[mrotteveel]

Similarly, the current determination of the "fractions" of time can also be simplified.

[mrotteveel]

I'm expanding the scope of this ticket to overhaul and cleanup the datetime handling, to try and express as much as possible in terms of java.time units, and push java.sql.Timestamp, java.sql.Time and java.sql.Date (and Calendar) to the edges.