Improve cleanup on lifetime scope disposal

[tillig]

Based on notes in this StackOverflow question, it seems LifetimeScope doesn't clean up after itself as well as it could.

For example, after calling Disposer.Dispose() to clean up IDisposables in the scope, should we also call _sharedInstances.Clear() to remove references from the shared instances?

Are there other places we're inadvertently holding references longer than needed?

[OnurGumus]

Can we back port this change to 3.5.x as well ? We are waiting .NET Core to be more matured.

[tillig]

@OnurGumus The 4.x series of Autofac is not just for .NET Core. If you want the updates, you'll need to upgrade.