lib: set long-running plugin workers threads to idle IO class

As outlined in https://github.com/flatpak/flatpak/pull/2071, running the threads which do intensive IO work (installs, upgrades, downloads) at SCHED_IDLE class significantly reduces disruption on the user's interactive tasks while the software updates take place. This effect is beneficial with the default CFQ scheduler, but most pronounced with BFQ where the mean and worst cases are returned to match those if no software install is taking place.