add toEpochSecond methods for efficient access

To efficiently get an epochSecond value:

- add 'toEpochSecond(ZoneOffset) ' methods to 'LocalDate' and 'LocalTime'.
- add 'toEpochSecond() ' method to 'OffsetTime'.

The method on 'LocalDate' would be defined to set the time part to midnight (safe as ZoneOffset, not ZoneId)
The methods on the time classes would be defined to set the date part to 1970-01-01.