Current refresh_pattern does not work for large debs

[Thomas Cuthbert]

The current refresh_patterns for ubuntu-repository-cache don't allow for squid to fetch large upstream debs while still serving from cache.

# refresh pattern for debs and udebs
refresh_pattern deb$ 129600 100% 129600
refresh_pattern udeb$ 129600 100% 129600
refresh_pattern tar.gz$ 129600 100% 129600

# refresh stats: https://pastebin.ubuntu.com/p/N9NdgWfrGk/

HTTP histogram:
Count %Total Category
 6571 0.18 Stale: expires time reached
     0 0.00 Fresh: refresh_pattern last-mod factor percentage

Cache Digests histogram:
Count %Total Category
  8293 1.65 Stale: expires time reached
  3672 0.73 Fresh: refresh_pattern last-mod factor percentage

On Store histogram:
Count %Total Category
  2352 11.26 Fresh: expires time not reached
     0 0.00 Fresh: refresh_pattern last-mod factor percentage

For example archive.ubuntu.com/ubuntu/pool/restricted/n/nvidia-graphics-drivers-470/libnvidia-gl-470_470.63.01-0ubuntu0.18.04.2_amd64.deb, is 138MB, it's a popular deb and once the refresh timer of 24 hours expires squid's LM-factor is not able to serve semi-stale content while it fetches upstream. https://pastebin.ubuntu.com/p/tNNXDf8c25/

I think:

refresh_pattern deb$ 129600 100% 129600
refresh_pattern udeb$ 129600 100% 129600
refresh_pattern tar.gz$ 129600 100% 129600

Should be:

refresh_pattern deb$ 129600 98% 129600
refresh_pattern udeb$ 129600 98% 129600
refresh_pattern tar.gz$ 129600 98% 129600

So squid has time to fetch larger debs upstream before the Cache-Control: max-age=$secs is reached.