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.
Maybe need something more tricky to avoid lag 0...1s on mode changes. I.e. send on status updates OR at 1Hz. Also need to disable periodic publication of vehicle_status topic in commander, only publish it on changes.