Improving performance of SceneCache SOP.

8.0.0-b2 introduced some code which improved performance when loading large hierarchies for the first time. This unfortunately caused a regression in performance when animating these hierarchies. This commit brings the animated perforamnce back inline with 8.0.0-b1, while maintaining (if not improving) the on-load performance from 8.0.0-b2.

Here are a few stats from a particularly heavy asset:

Load first frame
b1 10m 13.2s
b2 3m 41.9s
b3 7.26s

+1 Frame
b1 3.89s
b2 5m 17.9s
b3 5.1s

And here are a few stats from a standard character:

Load first frame
b1 1.35s
b2 0.28s
b3 0.48s

+1 Frame
b1 0.2s
b2 0.44s
b3 0.2s