Investigate using Unity build to speed up CV build time

CMake 3.16 (Nov 2019) natively supports unity builds - where multiple source files are combined into a single compilation unit for faster compilation times.

Speedups are claimed to be of the order of 3-4x.

We should investigate unity builds for CB Server to see how much of a speedup they give us. If it's a significant gain we should look at enabling unity builds for CV jobs.