Keep parts of data directory in /var/cache instead of /var/lib

Roman Mamedov points out in https://lists.torproject.org/pipermail/tor-relays/2017-November/013541.html that we keep huge amounts of volatile data like consensus diffs in /var/lib/tor by default. I have just yesterday run into the same problem as him and needed to write exclusion rules for backups to keep the size sane.