Generate W3C-compatible operation id if there is no request-id header

[lmolkova]

See microsoft/ApplicationInsights-dotnet-server#952:

We are going to gradually enable W3C support in ApplicaitonInsights.

While the details and backward compatibility conditions are still not clear, we want to make sure that customers have enough time to transition to W3C format and this change is the first step in this transition.

This change generates Activity RootId in the format of W3C traceId when there are no parents.

I.e. if the service has received Request-Id (or legacy) header from upstream, it will use it, no matter which format it follows.

However, if there were no such header in the incoming request, the new operation id will comply with W3C spec for traceId.

This is a temporary workaround that could be removed once W3C supported in .NET (Activity).

Sampling tests shows that distribution of sampling score for the new traceId generation algorithm is uniform.

This change should be merged after DependencyCollector beta4 is onboarded (and copy-paste code is removed)

[lmolkova]

I've tried to remove this guy and use one from Web SDK StringUtilities. Did not work out: https://github.com/Microsoft/ApplicationInsights-dotnet-server/issues/956, so we'll have to live with copy-paste for a while...