Use Duration instead of SystemTime in trace

[owickstrom]

The trace, or rather a metadata structure/file about the entire test case, should hold a start timestamp. Other than that reference, everything in the trace should be encoded as offsets from the start using Duration, not with full SystemTimes.

[owickstrom]

I've partly migrated off SystemTime into a microseconds-based newtype struct called Time. But we could still do offsets as was the original plan here. Perhaps when we introduce the test metadata file that can span multiple traces.