Removed the `Dbal` prefix from the class names and moved them to the `Doctrine\Tests\DBAL` namespace.

The `PostgreSqlPlatform`, `SQLAnywherePlatform` and `SQLServerPlatform` classes have been renamed to `PostgreSQL94Platform`, `SQLAnywhere16Platform` and `SQLServer2012Platform` respectively.

When deprecating an old platform version, we can deprecate and later remove a specific class. Otherwise, there's no way to express the deprecation on the class level and the upgrade notes contain sentences like: "`XYZPlatform` and `XYZKeywords` now represent XYZ (version)".

When the connection is lost or is closed, subsequent transaction will no longer be nested because they started in a brand new session. Our internal representation of the nesting shold take this into account