CommitMapChange doesn't fully deal with sub-levels

Per https://udn.unrealengine.com/questions/329206/crashes-when-streaming-levels-in-414.html

When inspecting the code for this report, it has become clear that the streaming levels of the incoming map are not getting updated correctly.

The StreamingLevels probably need to be renamed and moved to be owned by the new world and FakeWorld->StreamingLevels should be emptied.

See https://udn.unrealengine.com/questions/329206/crashes-when-streaming-levels-in-414.html