Correct cache setting behavior, man was I sleep-deprivated
Invalidate cache after "expiry", not every "expiry"

This took me quite some time to figure out. It was implemented by the
Mozilla guys, so I had no idea it worked this way.

I was seeing Latency spikes every hour, and they seemed right, a lot of
popular sites that need recaching, I was thinking.

Then I discovered a bug, I was writing to the cache every time. Fixed.
But wait, then why were there spikes at all?

Eventually I noticed that they are not merely every hour, but at the
beginning of every hour!

Here is the gotcha: they were using Truncate, invalidating all the
entries at the start of the period, so a entry from 11:59 will be
invalid at 12:00.

Changed.