postgresql / db-koji01: allow adjusting max_workers and friends
    
    db-koji01 has been running with this since before the mass rebuild, and
    it seems to make it have a higher load, but process faster and without
    stalling when doing backups or when long/bad koji-gc queries for old
    versions of texlive hit it.
    
    Signed-off-by: Kevin Fenzi <kevin@scrye.com>