pgsql: Simplify and encapsulate tuple routing support code.

Simplify and encapsulate tuple routing support code.

Instead of having ExecSetupPartitionTupleRouting return multiple out
parameters, have it return a pointer to a structure containing all of
those different things.  Also, provide and use a cleanup function,
ExecCleanupTupleRouting, instead of cleaning up all of the resources
allocated by ExecSetupPartitionTupleRouting individually.

Amit Khandekar, reviewed by Amit Langote, David Rowley, and me

Discussion: http://postgr.es/m/CAJ3gD9fWfxgKC+PfJZF3hkgAcNOy-LpfPxVYitDEXKHjeieWQQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/cc6337d2fed598d4b5ac54d9a62708182b83a81e

Modified Files
--------------
src/backend/commands/copy.c            |  86 ++++++--------------------
src/backend/executor/execPartition.c   | 108 ++++++++++++++++++++-------------
src/backend/executor/nodeModifyTable.c |  94 +++++++++-------------------
src/include/executor/execPartition.h   |  47 +++++++++++---
src/include/nodes/execnodes.h          |   9 +--
5 files changed, 154 insertions(+), 190 deletions(-)