Speed up hydra-update-gc-roots

The hydra-update-gc-roots script is taking around 95 minutes on our Hydra instance (though a lot of that is I/O wait). This patch significantly reduces the number of database queries. In particular, the N most recent successful builds for each job in a jobset are now determined in a single query. Also, it removes the calls to readlink().

Created by  Eelco Dolstra  on March 7, 2012
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