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().
The underscores are ugly and the .pl extension is an implementation
detail that shouldn't be visible to the outside.
Also, get rid of the *.in files. It's not really necessary to
generate them. And I was always modifying the wrong file.
