We've found subtle errors if you don't use Johnny Cache

Not sure if you'll want this patch in it's current form but thought a pull request was the best way to bring it to your attention.

Basically - without the Johnny Cache backend, the cache generation key gets reset a lot which leads to hard-to-spot cases where stale pages are served (as the key loops through a small set of values - older pages are still in the cache when it gets reset).

You do recommend and document this but it's really easy to forget or miss the problem so we decided it was too risky to allow Jimmy Page without a cache backend that allows infinite values.