Reduce code duplication in db.cpp, use builtin formatting in spdlog when logging, remove unnecessary usages of empty C-strings

[undertheironbridge]

db.cpp had some very repetitive code related to managing the query buffers. This is now handled by a local wrapper for std::string which provides the necessary functionality. Also, usages of "" were replaced with equivalent functionality provided by std::string member functions where applicable, mostly in context of object construction (where it was completely removed, as the default constructor does the same thing) and checking if a string is empty.