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.
This reverts commit 71d020735b.
Unfortunately there are still some cases where we need to set Hydra's
concurrency separately. (Ideally, Hydra would start *all* queued
builds in parallel and let Nix take care of everything...)
