Prior to this commit, the TSC frequency was calibrated at boot time by
measuring the TSC before and after a one-second sleep. This was simple
and effective, but had the disadvantage of *requiring a one-second sleep*.
Rather than making two measurements of the TSC (before and after sleeping)
we now perform many measurements; and rather than simply subtracting the
starting count from the ending count, we calculate a best-fit regression
between the TSC and the reference clock. While we do this, we keep track
of an estimate of the uncertainty in the regression slope (aka. the ratio
of clock speeds), and stop measuring when we believe the uncertainty is
less than 1 PPM.