- improved/changed image caching system
  * switched from btrfs to overlayfs
  * no loop mounts involved anymore, thus one layer less
  * using native kernel overlay filesystem which performs
    better than fuse based filesystems. btrfs has turned out
    to work unstable when using loop devices and I could not
    find a noticeable performance difference between
    btrfs seed devices and the overlayfs upper/lower system
  * no cache files but cache trees. full freedom for the user
    to store those data on NFS, tmpfs, read-only locations...
  * all package solver tasks now done via satsolver, solvables
    will also be cached, no hacky ignore of packages in the
    cache selection code
  * following the keep it simple principle. the caching code
    is now pretty straight forward