pgsql: Associate partitioning information with each RelOptInfo.

Associate partitioning information with each RelOptInfo.

This is not used for anything yet, but it is necessary infrastructure
for partition-wise join and for partition pruning without constraint
exclusion.

Ashutosh Bapat, reviewed by Amit Langote and with quite a few changes,
mostly cosmetic, by me. Additional review and testing of this patch
series by Antonin Houska, Amit Khandekar, Rafia Sabih, Rajkumar
Raghuwanshi, Thomas Munro, and Dilip Kumar.

Discussion: http://postgr.es/m/CAFjFpRfneFG3H+F6BaiXemMrKF+FY-POpx3Ocy+RiH3yBmXSNw@mail.gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/9140cf8269b0c4ae002b2748d93979d535891311

Modified Files
--------------
src/backend/optimizer/util/plancat.c | 159 +++++++++++++++++++++++++++++++++++
src/backend/optimizer/util/relnode.c | 37 +++++++-
src/include/nodes/relation.h | 56 +++++++++++-
3 files changed, 249 insertions(+), 3 deletions(-)