Make the NET_PARTICIPANT role dependent on user activity

This patch implements all of 28337, except for the part where we
turn off the role if we've been idle for a long time.

src/core/mainloop/mainloop.h

@@ -65,6 +65,7 @@ void reschedule_or_state_save(void);
 void reschedule_dirvote(const or_options_t *options);
 void mainloop_schedule_postloop_cleanup(void);
 void rescan_periodic_events(const or_options_t *options);
+void schedule_rescan_periodic_events(void);
 
 void update_current_time(time_t now);