Rename PathKeyInfo to GroupByOrdering

Rename PathKeyInfo to GroupByOrdering

0452b461bc made optimizer explore alternative orderings of group-by pathkeys.
The PathKeyInfo data structure was used to store the particular ordering of
group-by pathkeys and corresponding clauses.  It turns out that PathKeyInfo
is not the best name for that purpose.  This commit renames this data structure
to GroupByOrdering, and revises its comment.

Discussion: https://postgr.es/m/db0fc3a4-966c-4cec-a136-94024d39212d%40postgrespro.ru
Reported-by: Tom Lane
Author: Andrei Lepikhov
Reviewed-by: Alexander Korotkov, Pavel Borisov