I have no strong opinion about a specific cutoff time and am happy to change this PR to 10 days.

But note, that cutoff only affects users who explicitly issue `brew cask clean`. Arguably, those users would find zero to be the cutoff of least surprise, which I also believe is more consistent with Homebrew.  Though I stopped following the Homebrew source at a certain point.

My main purpose is to get the tracker symlinks in there, which then enables us to iterate on any type of cleanup behavior/interface going forward.