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)
Haven't heard of this one :) But to be consistent in that case you need to use "this" to call non-static methods (e.g. GetValue, SetValue) as well, right? And wouldn't that add lots of clutter?

(not that anything here bothers me that much and in any case it's your code)