Change support for custom begin scope

[ipjohnson]

Currently lifetime scope creation logic could be overridden by providing a ILifetimeScopeProvider. This introduces some extra over head for a very niche case.

To still support this I'm going to mark the BeginLifetimeScope method as virtual and you can override it if this is something that is needed.

[ipjohnson]

This is implemented and will be part of 7.0