Deduplicate time unit conversion factors [v9_18]
The various factors like NS_PER_MS are now defined in a single place and the names are no longer inconsistent. I chose the _PER_SEC names rather than _PER_S because it is slightly more clear in isolation; but the smaller units are always NS, US, and MS.
Backport of !7033 (merged)
Edited by Michał Kępień