First cleanup of NpgsqlConnectionString
- marked dead code (not removed yet)
- re-structured code file, easier to predict where things are
- moved enums / delegates to separate files, so less clutter in main file and again it's predictable where they are
- overall clean up of code: added comments where needed, added {} around single-line if statements
For my Npgsql Codebase Cleanup

- Removed dead code: private classes NpgsqlConnectionStringKeywordAttribute and NpgsqlConnectionStringAcceptableKeywordAttribute have been removed from NpgsqlConnectionStringBuilder as the attributes were only used inside the class but only as annotations on properties, not for any other purpose (no code reflected on them). As they're private, no code outside npgsql will use them either, and no code in npgsql uses them -> dead code -> removed.