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

Implemented comparison of default values as strings regardless of their PHP types

Fixes #3336.

Travis CI tests on PHP 7.3