Added conditionals to guards to remove them from release builds.
[PERFORMANCE] Parallelized GameObjectManager.ActiveObjects.

This will probably end up being slower for low numbers of game
objects, but in the thousands, it's quite a bit faster.
[PERFORMANCE] Renderers and components no longer flushed in cleanup

Modified Scene.RunCleanup so that it doesn't search through the list of
renderers and components for null or disposed items. It makes a real
difference in performance when dealing with thousands of items but note
that this potentially will leave null items in the lists to hang around until
the scene changes.