Hit ratio dropped significantly after recent upgrades

[Dridi Boukelmoune]

> To reiterate on a point in another of my responses in this thread, I think it may be something about MediaWiki thumbnail images not being cached properly despite our current VCL in that regard not having changed from how it worked prior to the upgrade during which time we were seeing a very high (86%-ish) hit ratio from the same formula.

To reiterate on a point I made on a couple occasions, it's time to
give varnishlog a spin. Too much focus on VCL, and not enough on
what's happening.

Dridi