wrapround of thread delays

wrapround of thread delays

  * made the wrapround of the underlying O/S occur before the wrapround
    of the delayed threads by making threads delay in microseconds since
    O/S epoch (1970 - Unix, 1601 - Windows) stored in Word64.
  * removed redundant calls reading O/S realtime clock
  * removed rounding to 1/50th of sec for timers
  * Only for Unix version of scheduler.