Update code and docs for generic hosting upcoming changes

Pull request aspnet/Hosting#1580 includes changes to how hosting works to support a generic host builder on which web host builder is layered. Some of these changes are breaking and we’ll need to adjust docs, examples, and code to support it.

• Adding an IServiceProviderFactory as a service no longer works. Call IHostBuilder.UseServiceProviderFactory instead. This changes the samples and renders the ServiceCollectionExtensions.AddAutofac method obsolete.
• You can no longer return an IServiceProvider from Startup.ConfigureServices. You must use the IServiceProviderFactory and ConfigureContainer. This will affect the use case where folks intentionally want to keep a reference to the built container or use a child lifetime scope as the pseudo-root of the app. I’ve mentioned that in a comment on the PR. You may need a custom service provider factory for this, but you can capture the factory instance during Configure.

In general it appears that while the new pattern moves toward a more generic hosting mechanism, it moves away from the notion of having composite application types (e.g., a self-hosted ASP.NET Core app and a background service all in one logical process).

[tillig]

Docs are adjusted and an RC package just went out. This will be the 5.0.0 release when it’s finalized, but we’ll wait on that final release until after .NET Core 3 itself is released.

[tillig]

I will likely release a beta package shortly. Also, don’t forget there’s a MyGet feed for early adopters where you can pick up and use any of our CI builds: https://www.myget.org/F/autofac/