crashDate having only per-day resolution was making client delay analysis
rather inaccurate.
crashTime is an ISO8601 string with per-hour resolution which should smooth
things over on the analysis side.
If per-hour is still too coarse, the use of an ISO string allows us to adapt
later to increasing the resolution, if it passes data review. The underlying
crash timestamp has per-second resolution.
MozReview-Commit-ID: 2hwJHSi8Xje
80d9deaf2e