sogrep: don't extract tarballs + automatically refresh cache if outdated

Created by: foutrelis

I think @eli-schwartz had some trouble with the auto-updating functionality when we briefly discussed this back in October(?). If I'm remembering correctly, that was caused by a broken mirror providing links databases that were 2-3 days old, thus causing sogrep to retry downloading them on each invocation.

The proposed changes seem to work well for me. Hopefully we can merge this because we can't (and IMO shouldn't) rely on people doing sogrep -r consistently enough.