Add worker sleep stats, rework stats machinery

The idea of the new stats machinery is that 'stats' becomes an option that enables the basic stats gathering infrastructure in EMPER. At some point, it should become a non-user option, i.e., it should be remove from meson_options.txt. Then comes a layer of fine-grained stats control switches, which default to 'auto'. Third, a new option called 'stats_all' is added, which, if enabled, activates all fine-graind stats knobs that are set to 'auto'.