WL#5972: Time limited privileges (based on a schedule)

Allow specifying an optional validity schedule when creating roles (or individul 
grants).
Should be very useful for environments where certain access is allowed to a group 
of users e.g. only during normal office hours.
Here's an example command :

GRANT ALL PRIVILEGES ON bank_accounts.* TO tellers SCHEDULE DAILY NOT BEFORE 
'09:00' NOT AFTER '18:00';