We now keep all builds in the N most recent evaluations of a jobset,
rather than the N most recent builds of every job. Note that this
means that typically fewer builds will be kept (since jobs may be
unchanged across evaluations).
This allows checking a jobset (say) at most once a day. It's also
possible to disable polling by setting the interval to 0. This is
useful for jobsets that use push notification or are manually
evaluated.
