Consider additional check for file age rather than parsed age

[astrobokonon]

The current setup (which is the same as the one Peter used) is to parse found directories names' and use that as their age, which has a pitfall:

If the code is turned off for a while, data is created in the meantime, and then the code is turned on after the rangeNew window has elapsed, the data could be silently ignored. I think there's a way to catch that scenario and trigger a retransfer, probably as part as Mandos' mantle of responsibilities when verifying that the machines are synced. But it needs testing and could be a looming bug around shutdown periods.

[astrobokonon]

Just realized that Mandos uses the same parsed-name-as-age logic as all the rest, so it seems like a modification date (or creation date if I give up on cross-platform) might be the only way to go.