[DDC-2919] Make lock mode usage consistent

This PR changes ORM's method signatures to declare a default lock mode of `null` instead of `LockMode::NONE` to make lock mode usage consistent and predictable on platforms supporting `LockMode::NONE` table lock hints (currently SQL Server and SQL Anywhere).
Please see `UPGRADE.md` and [DDC-2919](http://www.doctrine-project.org/jira/browse/DDC-2919) for further details.