AspNet: manage scopes in HttpContext.Current.Items

[ksjoberg]

This addresses the issue in #336.

There are several pieces that could/should be improved upon, perhaps mainly in the way in which the default IScopeManager is replaced. Up for discussion is also whether AspNetScopeManager should do the fallback the way it currently does it.

[lucaspimentel]

Thanks, @ksjoberg! Someone from the team will review this soon.

[colin-higgins]

Thanks for the contribution @ksjoberg
I added a test for web forms that verifies nested async calls work as we expect still.
Your changes have sparked some discussion here about how to handle what you mentioned with changing out IScopeManager and how the fallbacks should work.
I will be creating a PR to adjust our API for this soon.