pgsql: Redefine create_upper_paths_hook as being invoked once per upper

Redefine create_upper_paths_hook as being invoked once per upper relation.

Per discussion, this gives potential users of the hook more flexibility,
because they can build custom Paths that implement only one stage of
upper processing atop core-provided Paths for earlier stages.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/f1f01de145d0aaca80e6cf8b2ccb7e7f4ed1ad02

Modified Files
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src/backend/optimizer/plan/planner.c   | 45 ++++++++++++++++++++++++++--------
src/backend/optimizer/prep/prepunion.c |  7 +++++-
src/include/optimizer/planner.h        |  6 +++--
3 files changed, 45 insertions(+), 13 deletions(-)