Enable precompiled headers for KV-Engine build

Some experimentation with CMake 3.16's support for precompiled headers [1] shows that using them can reduce KV-Engine clean build time by at least ~15% - measuring using clang-build-analyser after precompiling a handful of popular headers[2]:

Before:

Compilation (742 times):
Parsing (frontend): 4798.7 s
Codegen & opts (backend): 728.7 s

After:

Compilation (744 times):
Parsing (frontend): 4093.3 s
Codegen & opts (backend): 726.7 s

We should finish off and integrate PCH support into the build.

[1]: https://onqtam.com/programming/2019-12-20-pch-unity-cmake-3-16/
[2]: https://github.com/daverigby/kv_engine/tree/clang_build_analyser_precompiled_headers