pgsql: Redo postgres_fdw's planner code so it can handle parameterized

Redo postgres_fdw's planner code so it can handle parameterized paths.

I wasn't going to ship this without having at least some example of how
to do that.  This version isn't terribly bright; in particular it won't
consider any combinations of multiple join clauses.  Given the cost of
executing a remote EXPLAIN, I'm not sure we want to be very aggressive
about doing that, anyway.

In support of this, refactor generate_implied_equalities_for_indexcol
so that it can be used to extract equivalence clauses that aren't
necessarily tied to an index.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/9cbc4b80ddc10b36c64514104caa69747c386dcf

Modified Files
--------------
contrib/postgres_fdw/deparse.c                 |  399 ++++++----
contrib/postgres_fdw/expected/postgres_fdw.out |   23 +-
contrib/postgres_fdw/postgres_fdw.c            |  941 +++++++++++++++---------
contrib/postgres_fdw/postgres_fdw.h            |   11 +-
contrib/postgres_fdw/sql/postgres_fdw.sql      |    4 +
src/backend/optimizer/path/equivclass.c        |   35 +-
src/backend/optimizer/path/indxpath.c          |   41 +-
src/include/optimizer/paths.h                  |   18 +-
8 files changed, 919 insertions(+), 553 deletions(-)