Forward declare all third party dependency in core code

[andyfengHKU]

This has been a known problem in our code base. Spdlog as a very big header-only library was introduced into buffer_manager.h and then indirectly being included in almost all files which causes the build time of each file to increase from 1s to 5s.

Now that we have forward declared Spdlog, there are still other third-party header-only libraries that are being used in a very similar way. nlohmann_json is an example. I was trying to forward declare it and it seems hard to do so (though a possible solution exists here nlohmann/json#700). Someone should try to solve this by either

• forward declare; or
• use our interval data structure in the core and only convert to nlohmann_json at the top-most level e.g. main.