QDateTime/QTime truncates milliseconds when converting to Qt::ISODate string

QTime uses QString::asprintf("%02d:%02d:%02d", hour(), minute(), second()); when converting to a string for Qt::ISODate, but ISO 8601 includes possible milliseconds.

This means that parsing a string into a QTime and outputting it again later will truncate milliseconds, potentially making two distinct events on the wire have the same timestamp when in reality they didn't.

HH:mm:ss.zzz -> fromString(Qt::ISODate) -> QTime(HH, mm, ss, zzz) -> toString(Qt::ISODate) -> HH:mm:ss

Workaround for 5.8 is to add a new enum Qt::ISODateWithMs that fixes the behaviour, without backwards-comparability issues.

https://codereview.qt-project.org/#/c/163360