Track the most recent upload time for duration events

This will allow us to see if images uploaded recently, which have been
subjected to thumbnail prerendering, load faster.

Change-Id: I4ce358ff54e4ca4e290349aa3ff4810ad4714d24
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/975
Merge "Track the most recent upload time for duration events"