org-clock: Improve `untilnow' range behavior and performance

org-clock: Improve `untilnow' range behavior and performance

* org-clock.el (org-clock-special-range): Set `untilnow' to use the
year -50000, rather than the earliest representable date.
(org-clock--oldest-date): Remove.

The `untilnow' range relied on the constant `org-clock--oldest-date`
to find the earliest representable date, which caused delays when
loading `org-clock' on systems where `most-negative-fixnum' is large.
This change removes that constant in favor of a simpler hack to
produce a range between the current time and before the dawn of human
civilization.  If this breaks your workflow, please report to the Time
Police.

TINYCHANGE