Change AutofacServiceFactory: more new

[marcrocny]

Submitted more as a commentary/reference to PR #10, this is a sketch of a more "consistent" version that replaces the function of AutofacServiceFactory outright.

Changes service-factory to return self if IServiceFactory is requested, and new-up a scope-factory when requested, rather than resolve. Also, don't resolve IServiceFactory in other contexts where new will suffice.

[tillig]

As noted in PR #10 I think we'll add some extensions to Autofac.AspNetCore and include the StackOverflow workaround in an easy to consume way in that package. More details in #10. Huge thanks for taking this time and helping us think through the issue!