This makes CConfigDB into a regular class. Not only was CConfigDB a singleton, it had only static members, making it effectively unique.
This was quite annoying in tests, as I discovered, and is just a horrible idea in general, since the members can actually outlive the singleton.
This updates config Hooks to be RAII types, simplfying client code.
It also makes it possible to load several config files at the same time, which would be of interest for D3593.