Improve handling of CustomPath/CustomPlan(State) children.

Robert Haas <rhaas@postgresql.org>

Commit: 5ca611841bcd37c7ee8448c46c8398ef8d8edcc4
Author: Robert Haas <rhaas@postgresql.org>
Date: 2015-06-26T13:40:47Z
Releases: 9.5.0
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.

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