Support PostgreSQL 14

Support PostgreSQL 14

In PostgreSQL 14, planner was restructured in regard to query string
passing.  This change allows us to remove fair amount of lines and
complexity that had been added as a struggle for retrieving proper
query string.

Two callbacks, post_parse_analyze_hook and ProcessUtility_hook are no
longer used.  get_query_string is removed altogether.  Also
current_hint_retrieved juggling, which was complex and fragile,
disappears. Now pg_hint_plan_planner alone can retrieve hint string by
looking into the given string.

In PostgreSQL 14, a new optimization item "memoize" is added. It is
not handled in this commit and leave it for a later commit.