Added "string" mapping in SqlitePlatform

  - fix 'Unknown database type string requested, Doctrine\DBAL\Platforms\SqlitePlatform may not support it.' error

Use a wrapped PDO Statement instead of a custom class for `pdo_sqlsrv` since `PDOSqlsrv\Statement` doesn't extend `PDOStatement` anymore.

The regression was introduced by 2b8c40d267b4ac699ea878dcca07332e1697978c.

Fixes #3060.

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