d-f-s: compute a score for the images

the current code takes the total count of usage for each image since its origin. This is great to not remove an image that is often used, but can not handle 2 cases:

• the image has not been used for a while but it was used quite a lot in the past
• the image is freshly backed and has been pulled only a couple of time

We should likely provied a score instead of a plain count. In https://gitlab.freedesktop.org/freedesktop/fdo-containers-usage we provide the last usage. We should use that as a parameter to a function that makes:

• images used in the last 24h should have a big score
• the longer the image has not been used, the smaller the score should be

Something like math.exp(1/x) might be a good candidate