Use subdirectories for job queue archive

As it turned out, having many files in a single directory can be
very painful. With this patch, only 10'000 files are stored in a
directory for the job queue archive. With 10'000 directries, this
allows for up to 100 million jobs be archived without having large
numbers of files in a single directories. Not that it is realistic,
anyway.

Reviewed-by: ultrotter