First PR for Npgsql technical debt cleanup

I've started with the codebase cleanup. As a lot of the code is a bit chaotic due to all the changes done over the past years by various authors, it's best to clean the stuff up a bit, weed out dead code and the like, restructure files, make everything look the same and professional and more predictable, so it's easier to understand when reading the code. 

I've started with the connectionstring builder. I had cleaned up the connection class as well, but that was in v2.2, and as it changed a lot in v3 branch, I'll re-do that one later. 

(all tests pass)
It's redundant, but the reason it's here is that you can see immediately that ExplicitDefault isn't a static field, it's an instance property (also not an instance field). This is for when one is reading the code.