Allow synchronous update of timestamps in CLOCK log.

Allow synchronous update of timestamps in CLOCK log.

* org.el (org-shiftcontrolup, org-shiftcontroldown): New
commands to use `org-clock-timestamps-change'.

* org-clock.el (org-clock-timestamps-change)
(org-clock-timestamps-down, org-clock-timestamps-up)
(org-at-clock-log-p): New functions to let the user update
clock log timestamps while keeping the same clock duration.

Thanks to Rainer Stengele for this idea.