Sample TrueTime and use that to build a clock drift estimation.

Sample true time and use the samples to build a linear estimation of clock drift, which is then applied at every call for the current "best time". This provides the app with continuous best-estimate of the actual time that is calibrated by TrueTime and corrected for clock drift.

Credit for this idea entirely goes to [Chris from UMass](https://www.geo.umass.edu/people/chris-duncan). He also was kind enough to send a sample of the code he's using for this idea.

Working on trying to integrate this as part of TrueTime.