Extend HistoricItem to work with Instant instead of ZonedDateTime

[joerg1985]

This PR will add a new default method to the HistoricItem interface, to get the timestamp as Instant.
The most areas using the HistoricItem must transform the internal Instant to a ZonedDateTime (e.g. all the persistence implementations) or need to transform the returned ZonedDateTime to an Instant (e.g. see the below).

So i my mind it does make sense to use a Instant here and not transform the data.
The link to the persistence implementations above does show the changes needed to the addons.

Signed-off-by: Jörg Sautter [email protected]

[rkoshak]

Will this be a breaking change for rules which currently expect a ZonedDateTime?

[joerg1985]

@rkoshak no this is a none breaking change, the new method has a default implementation and all old methods are still in place.