Moved the helper class to the public scope for reuse outside the optimizer.

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/trunk@59333 65644016-39b1-43b1-bf79-96bc8fe82c15
Fixed CORE-4379: Poor performance of explicit cursors containing correlated subqueries in the select list.

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/trunk@59334 65644016-39b1-43b1-bf79-96bc8fe82c15