support SQL-standard date/time functions

This allows setting date, time, and datetime to the values `CURRENT_DATE`, `CURRENT_TIME`, `CURRENT_TIMESTAMP`, `LOCALTIME` and `LOCALTIMESTAMP` from the API, and translates that into SQL expressions that get evaluated by the database.

This allows client-side code to set a time, date or timestamp to current as seen by the server, in case the client computer's clock is incorrect.