distinguish between jobs with the same name in different jobsets
(e.g. "trunk" vs "stdenv-branch" for Nixpkgs).
* Renamed the "attrName" field of Builds to "job".
* Renamed the "id" field of BuildSteps to "build".
Note: to upgrade old databases, do a dump with an old Sqlite first;
dumping with a new Sqlite will silently discard (!) the contents of
the ReleaseSetJobs table.
failed in a previous build. This is essential for Nixpkgs: we don't
want to keep doing the same failed dependency (say, Glibc) over and
over again for a few hundred jobs.