#9 priority level for cache stores

[Joshua Pritikin]

I want to segregate cache stores by the importance of
the data. For example, I want to cache all debian
packages in a special store. Everything else I want to
cache in another disk store. This makes it easy to
preserve the important store (debs) if I need to
reorganize my disk partitioning.

For implementation, add a priority level to stores
which is consulted before considering the cache balance
method.

If you are feeling lazy then I can submit a patch for
the same. Let me know.