Replace SceneAdder 100ms sleeps with sleeps only as long as needed

[workhorsy]

Adding scenes is slow. At the current rate (100ms), we can only add 10 scenes per second. This makes it seem very slow when adding tons of small scenes.

[workhorsy]

Instead of using an arbitrary amount of time to wait. Just have the target tell the thread after it has added the instance. This seems to work great for the shader cache.