Watchdog on DCC-EX protocol connection

[habazut]

Need to implement a set watchdog command on the DCC-EX protocol so that throttles can request that this connection is watched for update within a certain hearbeat interval and if the hearbeat does not happen some action (like stop of engine(s)) is taken.
The watchdog command should be able to set some timer. To be investigated how or what actions can be specified.

Feeding the dog withing time (with margin) will be then the responsibility of the throttle. (DHCP for example renews 50% before timeout)
It is the throttle that knows if the connection is important enough to be watched.