Reconsider hinting process

Reconsider hinting process

I rethinked the steps of enforcement considering the parallel planning
code. pg_hint_plan_set_rel_pathlist() gets simpler and easier to read
(maybe).

As the result of this patch, some behaviors has been changed.

- False generation of gahter path caused by hinting on the other side
  of joins.

- Scan hints work with Parallel hints. (It's useless for the moment,
  though).