Add option to remove old unused versions of packages.

[p2004a]

This is to help reduce amount of storage/packages that need to be scanned and loaded by spring, by allowing to remove all packages from e.g. byar:* repo, except for some selected tags.

[Beherith]

So, this is a rather complex beast, I have no idea if this is feasible at all while the packages are loaded by spring-dedicated, or unitsync.
But since hosts tend to have a 'range' of active versions running, I will first have to check if cleaning up the cache even reduced SPADS memory footprint before asking for stupid features.