Add Activity-tracking to correlate events from specific iterations

[analogrelay]

Each iteration causes an IterationStart and IterationStop message to be emitted. On .NET 4.6 (and hopefully CoreCLR, though I will check) this means an Activity will be allocated for the Iteration (as per http://blogs.msdn.com/b/vancem/archive/2015/09/14/exploring-eventsource-activity-correlation-and-causation-features.aspx). All ETW events that take place between the start and stop messages will get this activity ID and thus can be correlated back to the original iteration. We should set up collectors to support this.