Fix for [DDC-2506] CTI JOINs and WITH conditionals

http://www.doctrine-project.org/jira/browse/DDC-2506

Moved `SqlWalker::walkConditionalExpression()` out of `walkJoin()` and into `walkJoinAssociationDeclaration()` after the base class JOIN but **before** `_generateClassTableInheritanceJoins()` thus fixing WITH conditions in *Class Table Inheritance* JOINs.