Detect and correct for clock skew

[phord]

After we run a couple of hours we should have a pretty good idea of how wrong our internal chronometer is. Calculate the wrongness and adjust our operational speed accordingly. For example, if we find that two hours have passed on our internal counters but the NTP server indicates 2 hours and 12 seconds have passed, we know we are running slow. We need to increase our speed by 0.17%, for example by adding one second every 600 seconds.

To start with, we can just calculate the skew and report it. Useful to verify steady calibration over time. Later we can add actual skew adjustment.