[DBAL-669] Make schema visit namespaces

This PR is a followup to https://github.com/doctrine/dbal/pull/444 and fixes schema not visiting namespaces.
Because of this issue the ORM test suite is [currently failing](https://travis-ci.org/doctrine/doctrine2/jobs/32975659). The schema tool is not creating the namespaces before actually creating the namespace prefixed tables, therefore one test case is failing in ORM.
With this PR now the `CreateSchemaSqlCollector` is also generating the appropriate namespace creation SQL.