Representing a history of status changes

[Pavinati]

I'm trying to model the status of a device while also keeping track of its past values. This concept is something in between a property and a datastream, but neither of those fit my user case well enough. Properties don't have a history, so i can't retrieve the previous values. Datastream are more suited for "events" or single datapoints. I could model the problem with a datastream that tracks the status changes but there is no clear API to retrieve the device status in a given time interval, as using since=X and to=Y will not return which status was active at the timestamp X

[Annopaolo]

This may be related to #591 too