[rayon] Thread-safe cache

[dannymcgee]

This PR updates the cache module to be thread-safe. I added some timers to the complexmap example, and these are the results I got vs the develop branch (on a Ryzen 7 1700x 8-core/16-thread @ ~4GHz):

develop		rayon
Unscaled	8m 58s	Unscaled	1m 18s
4x	9m 25s	4x	1m 17s
16x	9m 29s	16x	1m 22s

Not the most scientific test I know, but the difference is pretty extreme. :)

Let me know if you'd like any changes to this. Also, I'd be happy to resolve conflicts with develop and open a PR into that branch as well if that's something you would be interested in, just let me know.