Allow configuring a watchdog for monitored items

[marcschier]

We have seen situations where a server does not report data for an item even so it is changing in the backend. But the subscriptions are good and keep alives arrive continuously, just no data change notifications.

We would benefit from a setting that monitors a value change arriving on a monitored item within a configured watchdog timeout (a counter that is a factor of max(sampling interval, publishing interval)) and then reset the subscription, session or reset the publisher entirely.

A default watchdog that could be set up here would be to monitor the server time and reset when the time changes are not received within e.g. 3 * (sampling interval == publishing interval == 1 second).

[marcschier]

#2256