[PATCH] bug 2487 cache_dir min-size setting

I had to work from max-size already in 3.x rather than the 2.7 code
patches. So I'm not sure if there are any fancy things min-size did in
2.x beyond limiting where stored objects went on disk. This patch
leaves it to the basic yes/no store dir selection.

NP:
  I noticed while doing this patch that the global storedir_maxobjsize
which determines the delay size to accumulate before selecting a dir
does not account for RAM cache. That may be part of the problem(s) when
no disk caches are present.

Amos