Inconsistent clock for Timestamp property

[marcschier]

Timestamp property in each data set message is the PublishTime, except when the notification is generated, e.g., cyclic read or heartbeat, or key frames. Then it is DateTime.UtcNow. It should be of the same clock, so it is comparable.

A command line option should be added to switch between sending wall clock of OPC Publisher or using publish time (where available). If no publish time is available, no timestamp property should be part of the message (to have consistency here as well). Proposal --MessageTimestamp=PublishTime --MessageTimestamp=CurrentTimeUtc. This will enable us to add other timestamping options in the future. (e.g., local time, or mixed option like today, or maxSourceTimestamp, etc.)