datarace: factor subtle clock-vector logic into a reusable function

datarace: factor subtle clock-vector logic into a reusable function

These comparisons are performed many times throughout datarace.cc, featuring a
lot of code duplication without strong documentation. This factors the
duplicated code into a single function and documents that single function more
clearly.

Brian Demsky: please review this fix (it should make no actual logical change)
and take a look at the TODO I have inserted; I'm not sure if the TODO is
warranted.