Runtime partition pruning for MergeAppend

Back in the v11 cycle, there was just not quite enough time to get the
MergeAppend run-time partition pruning patch in.

I've attached v24 of this patch.  The only changes done from v23 [1]
are to re-base the patch atop of current master. There's was a bit of
churn in the partition pruning and run-time pruning code last week.
I've re-aligned the new code to account for the work done there.

I'll add this to the July 'fest.

[1] https://www.postgresql.org/message-id/CAKJS1f_yFC8vVLd3mHP4pxhRgPRKb9hqa4Zv+5v5pko6f038dA@mail.gmail.com

-- 
 David Rowley                   http://www.2ndQuadrant.com/
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