DDC-1385

[beberlei]

This branch does multiple things:

1. Fix how INDEX BY works with mixed results. The index is now on the top level, not inside the mixed result.

2. Add support for INDEX BY in scalar only queries.

   SELECT u.id, u.name FROM User u INDEX BY u.id
   

The following BC Breaks had to be made:

1. The hydration format changed when using INDEX BY with either mixed or scalar only results.
2. ResultSetMapping::$indexByMap is now $dqlAlias => $columnName, not $dqlAlias => $fieldName anymore.

Todos:

1. Since Hydration uses 'scalars' internally, we should prevent that identification variable name from being used.
2. Array Hydration scalar only is missing.
3. Unit-tests for RSM changes, aswell as some more error conditions (Exceptions).
4. Scalar only array hydration for nested levels?

Please code review.