.NET Standard compat: Avoid usages of CallContext

[jdom]

In our .NET Standard solution I temporarily removed the usages of CallContext from our RequestContext class, as it's not supported in .NET Standard. See 772dde7 for the commit where this was excluded.

We need to find an alternative. Without too much investigation on my end, it seems that the recommended alternative is to use AsyncLocal<T> which is available in both .NET Standard and .NET 4.6+. If we go with this approach, we should probably keep this code under conditional compilation for the mean time, as we are not ready to update the dependency for the main solution (non-vNext) to 4.6 just yet (soon though).

In order to open the solution that compiles using .NET Standard (and hence easy to check what is available in that environment), check out this readme