DRILL-7926: Age Function Fix (#2284)
* Fix query start time code and Drill timestamp util functions.
* Implement AGE function as a Freemarker template.
* Add tests for the unary form of AGE.
* Use Java Time's LocalDateTime.until to simplify TIMESTAMPDIFF.
* Fix expressions in the test physical plan in age.json.
* Add comments explaining AGE function logic.
Co-authored-by: Oleg Zinoviev <ozinoviev@solit-clouds.ru>
