Extend time information of `EventContext`

[alexander-yevsyukov]

Currently, EventContext only contains information about the time-zone of the Origin of the event. Presumably most frequent example of this is the time-zone of the user who created the command which resulted in the event. It could happen, though, that the event is occurred somewhere in another time-zone, and this information may be valuable for further analysis.

Another aspect timing of events is to differentiate when the event is generated and discovered by the system. See on Martin Fowler writing on this.

I don't think we have usage scenarios in the projects that use Spine yet. But such a property may be important for deciding, for example, if an event is worth handling. Another case may be measuring latency of delivery of events by the transport.