This callback is called when the first heartbeat is received (after BOOTUP)
and when there was a pause between heartbeats (TIMEOUT). This allows
the user to get an event when a bus segment is disconnected and then
reconnected from the server (nodes just start sending heartbeats again
without BOOTUP first).
HB emergency was only set but never cleared, so only the first error ever
was reported. We now get an emergency when the first node is missing and
a clear emergency when the last node turns up again.
The same applies for HB Remote Reset, however while clearing this emergency
is required by the CO_Emergency module, it's not required for the network.
