Fine grained caching for large files

[andrewchambers]

The current bupstash send log stat cache does caching on a per directory basis, this works great for many small files and gives a huge speed boost, however this is not great for directories like photo albums where there are many large files where the current implementation must rehash all files each time a new photo is added.

If a directory has many large files, it makes sense they are cached individually instead of grouped.

[andrewchambers]

related to #314