Heartbeat sending enforce

I found a rare condition where the status topics wouldn't update together in HIL, leading to the system not sending constantly heartbeats. This patch

  - Enforces a minimum rate configuration of 1 Hz
  - Always sends a heartbeat, independent if the topics updated

Inter-system (onboard) links also should send heartbeats, so special link modes still work as specced.
@DrTon That sounds fair, but something for a separate PR once we finished discussing this one.