[WIP] Soft-migration from Zeta Components Database to Doctrine DBAL

This Pull Request is a first prototype for migrating to Doctrine DBAL as a database abstraction layer instead of Zeta Components Database.

The `doc/specificiations/refactorings/doctrine-dbal.rst` contains an overview of the work to be done. Refactoring option 2 was chosen, where a Zeta Components compatible Query API is developed to avoid rewriting all the gateways. The prototype introduces a new `eZ\Publish\Core\Persistence\Database` namespace with interfaces for this abstraction. The Doctrine implementation is located in `eZ\Publish\Core\Persistence\Doctrine` and contains a limited set of tests (could be improved).

The Gateways have all been adjusted to use the new handler abstraction and implemented as much that the integration tests pass on SQLite.

Tasks:

- [x] Fix `eZ\Publish\Core\Persistence\Legacy\Handler` to directly accept a `\Doctrine\DBAL\Connection` instead of the `EzcDbHandler`. This requires changing the testcases that construct database connections. As Doctrine does not have a syntax for creating a connection from a DSN, this code has to be ported.
- [x] Fix tests on all platforms
- [x] Exception wrapping Gateways should convert `PDOException` to `Doctrine\DBAL\DBALException`
- [x] Docblocks of ezcQuery and ezcDb should be fixed to `eZ\Publish\Core\Persistence\Database` namespace.
- [x] File- and class-level docblocks of some of the new files are missing.
- [ ] Rename `EzcDatabase` gateway classes to `DatabaseHandler` or `DoctrineDatabase`
- [ ] Integration into Symfony DIC

SQL-Ng related:

- [ ] move Doctrine Exception Conversion to the `eZ\Publish\Core\Persistence\Doctrine\Gateway` namespace for re-use with Sql-Ng
- [ ] Sql-Ng branch should be adjusted (part of its own PR)

Tasks for external reviews:

- [ ] Evaluate the change of the `rightJoin` query to not use a RIGHT JOIN. Is this semantically correct? The right join seemed unnecessary, since the WHERE clause uses both tables.

More Todos forthcoming..