Report both actual storage disk usage, and size of files known to Librarian

[pkgw]

We're being bitten about ssh_prefix vs. path_prefix vs. rsync_prefix, etc. We'd also like to make sure that the disk usage of each store is correctly referenced to ground truth, so there should be some little cron job that does this kind of stuff.

[bhazelton]

we should separately report the disk usage of files the librarian knows about and the total disk usage.

[pkgw]

We've straightened out the "prefixes" and are correctly ground-truthing the disk usage, but it would still be nice to be able to compare that usage to what the Librarian thinks should be used. (This might get a bit tricky since the Librarian knows the number of actual bytes in each file, but not the actual disk space taken by each file, which will be different because files take up whole filesystem blocks, and directories have an overhead.)

[pkgw]

So far this feature has not felt that important to implement, but it would be fun to have.