Refactor ChangeVarNodesExtended() using the custom callback

Refactor ChangeVarNodesExtended() using the custom callback

fc069a3a6319 implemented Self-Join Elimination (SJE) and put related logic
to ChangeVarNodes_walker().  This commit provides refactoring to remove the
SJE-related logic from ChangeVarNodes_walker() but adds a custom callback to
ChangeVarNodesExtended(), which has a chance to process a node before
ChangeVarNodes_walker().  Passing this callback to ChangeVarNodesExtended()
allows SJE-related node handling to be kept within the analyzejoins.c.

Reported-by: Richard Guo <[email protected]>
Discussion: https://postgr.es/m/CAMbWs49PE3CvnV8vrQ0Dr%3DHqgZZmX0tdNbzVNJxqc8yg-8kDQQ%40mail.gmail.com
Author: Andrei Lepikhov <[email protected]>
Author: Alexander Korotkov <[email protected]>