[yang] Added date-and-time, keep backward compatibility for timestamp

Summary: Added date-and-time, keep backward compatiblity timestamp

Test Plan: Test all date-and-time types.

Reviewers: orixmann, mjosenhans

Reviewed By: druss

CC: mkraemer, druss, aschultz

Differential Revision: https://review.tpip.net/D1389
[yang] Added date-and-time, keep backward compatibility for timestamp

Summary: Added date-and-time, keep backward compatiblity timestamp

Test Plan: Test all date-and-time types.

Reviewers: orixmann, mjosenhans

Reviewed By: druss

CC: mkraemer, druss, aschultz

Differential Revision: https://review.tpip.net/D1389