App::CreateThread

[shuhaowu]

Instead of letting Thread be created outside of the app, we now force the Thread to be created by the App::CreateThread factory method. This allows us to better control lifetime and sets up the stage to possibly eliminate the weak_ptr situation with the trace aggregator if the Thread is not longer a shared_ptr.

Issues:

• Thread can be retained by the user as a shared_ptr which means it might not go out of scope before App does.
  • An issue because after thread finishes executing, it needs to notify the TraceAggregator which could have gone out of scope with the App.
• Technically there's still an issue where App goes out of scope before the Thread joins. The Thread currently would just be detached and it will try to notify the TraceAggregator when it finishes.