Minor bugfix and refactoring

Bugfix: validation - ambiguous column reference correctly recognized
when refering multiple columns of same name from subquery, e. g., SELECT
NAME FROM (SELECT S.NAME, RB.NAME FROM SELECTION S JOIN RESPONDENTBUCKET
RB ON S.RESPONDENTBUCKET_ID = RB.RESPONDENTBUCKET_ID)
Refactoring: OracleColumnReference - introduced back reference to
SelectListColumn
Refactoring: OracleQueryBlock - introduced AllColumnReferences property