Backport CCMA performance improvements

[jchodera]

The changes in openmm#2818 would also be super helpful for the COVID Moonshot.

[peastman]

I don't think that will be practical. That code is very different between 7.4 and 7.5, since it got completely overhauled while unifying the CUDA and OpenCL implementations. It will be a better use of our time to just focus on getting out a 7.5 based core.