pgsql: Move code for managing PartitionDescs into a new file,partdesc.

Move code for managing PartitionDescs into a new file, partdesc.c

This is similar in spirit to the existing partbounds.c file in the
same directory, except that there's a lot less code in the new file
created by this commit.  Pending work in this area proposes to add a
bunch more code related to PartitionDescs, though, and this will give
us a good place to put it.

Discussion: http://postgr.es/m/CA+TgmoZUwPf_uanjF==gTGBMJrn8uCq52XYvAEorNkLrUdoawg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1bb5e78218107db543086e71fc162d36a955b08e

Modified Files
--------------
src/backend/catalog/heap.c            |   1 +
src/backend/catalog/partition.c       |  16 ---
src/backend/catalog/pg_constraint.c   |   1 -
src/backend/commands/indexcmds.c      |   2 +-
src/backend/commands/tablecmds.c      |   1 +
src/backend/commands/trigger.c        |   1 +
src/backend/executor/execPartition.c  |   1 +
src/backend/optimizer/util/inherit.c  |   1 +
src/backend/optimizer/util/plancat.c  |   2 +-
src/backend/partitioning/Makefile     |   2 +-
src/backend/partitioning/partbounds.c |   6 +-
src/backend/partitioning/partdesc.c   | 221 ++++++++++++++++++++++++++++++++++
src/backend/utils/cache/partcache.c   | 124 -------------------
src/backend/utils/cache/relcache.c    |  57 +--------
src/include/catalog/partition.h       |  15 ---
src/include/partitioning/partdesc.h   |  39 ++++++
src/include/utils/partcache.h         |   1 -
17 files changed, 273 insertions(+), 218 deletions(-)