More ccache sizing tweaks
More ccache sizing tweaks

Summary:
The previous set of tweaks definitely helped. Bistro is now consistently more than 3x faster than before.

Even with the new doubling regime, Proxygen is having trouble getting a big enough cache -- it's a lot faster, but still many cache misses.

I thinky my silly mistake was operating in terms of percentages, while ccache most likely just has a threshold of a "100-200MB away from the max" when it triggers a cleanup.

So, let's try that approach here. I think it should better avoid undersizing caches than the current "2x of usage" tactic.

Reviewed By: sdwilsh

Differential Revision: D5705743

fbshipit-source-id: 5e9bc4cccc18ae6406ff09f54ff48a8bc5c9eedf

facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro