[BC] Replaced extension of \PDOStatement with a composition to avoid having to replicate the \PDOStatement interface in ResultStatement

Fixes https://github.com/doctrine/dbal/issues/3114

- Fixed docblocks
- Removed return statements on void methods
- Cleaned up unused private / local variables
- Fixed license issues
- Removed SVN leftovers
- Fixed doctrine-project.org links
- Added line breaks before return statements

When using a PDOConnection and the Portability wrapper, the default value \PDO::CASE_NATURAL int(0) and PHP constant CASE_LOWER int(0) got mixed up.

This enables a more natural way of interacting with result sets,
and re-introduces in Doctrine DBAL a feature that has been available
in PDO from the begining.

[DBAL-56] Add portability wrapping layer porting MDB2 functionality to fix case, rtrim strings and convert empty strings to null.