pgsql: Improve handling of CustomPath/CustomPlan(State) children.

Improve handling of CustomPath/CustomPlan(State) children.

Allow CustomPath to have a list of paths, CustomPlan a list of plans,
and CustomPlanState a list of planstates known to the core system, so
that custom path/plan providers can more reasonably use this
infrastructure for nodes with multiple children.

KaiGai Kohei, per a design suggestion from Tom Lane, with some
further kibitzing by me.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/5ca611841bcd37c7ee8448c46c8398ef8d8edcc4

Modified Files
--------------
doc/src/sgml/custom-scan.sgml           |   10 +++++++++-
src/backend/commands/explain.c          |   22 ++++++++++++++++++++++
src/backend/optimizer/plan/createplan.c |   13 ++++++++++++-
src/backend/optimizer/plan/setrefs.c    |    8 ++++++++
src/backend/optimizer/plan/subselect.c  |   25 +++++++++++++++++++++----
src/include/nodes/execnodes.h           |    1 +
src/include/nodes/plannodes.h           |    1 +
src/include/nodes/relation.h            |    4 +++-
8 files changed, 77 insertions(+), 7 deletions(-)