cache fills up with non-current catalogs

[mharvey-jt]

For repos that get very frequently update, the cache can end up full of non-current versions of high catalogs, eg, this instance has 117k versions of the root catalog:

cvmfs_talk -i REPO cache list catalogs | grep -e ":/ "   | wc -l
117279

since the cache manager is given the path of the catalog on insert, it would be advantageous if the insert replaced any existing version, rather than treating it as an entirely different entity.