RFC 235: Find equivalent of the now useless "freeSpace" operation in the janitor

Created by: efritz

Previously we stored all data on disk, which made it easy to stat to know our capacity and our used space. Now that we store data in Postgres, we need to know how to clean things out over time. But we can't as easily determine the size occupied by a single dump over several large tables. It may be a critical feature for some disk-constrained deployments with large uploads so we should do this before the 3.21 branch cut.