pgsql: Add a GetForeignUpperPaths callback function for FDWs.

Add a GetForeignUpperPaths callback function for FDWs.

This is basically like the just-added create_upper_paths_hook, but
control is funneled only to the FDW responsible for all the baserels
of the current query; so providing such a callback is much less likely
to add useless overhead than using the hook function is.

The documentation is a bit sketchy. We'll likely want to improve it,
and/or adjust the call conventions, when we get some experience with
actually using this callback. Hopefully somebody will find time to
experiment with it before 9.6 feature freeze.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/101fd9349eddb7e9ed84a239145d5230a9bc7336

Modified Files
--------------
doc/src/sgml/fdwhandler.sgml | 43 ++++++++++++++++++++++++++++++++---
src/backend/optimizer/README | 5 +++-
src/backend/optimizer/plan/planner.c | 16 ++++++++-----
src/backend/optimizer/util/pathnode.c | 14 ++++++------
src/include/foreign/fdwapi.h | 6 +++++
5 files changed, 67 insertions(+), 17 deletions(-)