Add readfuncs.c support for plan nodes.

Add readfuncs.c support for plan nodes.

For parallel query, we need to be able to pass a Plan to a worker, so
that it knows what it's supposed to do.  We could invent our own way
of serializing plans for that purpose, but piggybacking on the
existing node infrastructure seems like a much better idea.

Initially, we'll probably only support a limited number of nodes
within parallel workers, but this commit adds support for everything
in plannodes.h except CustomScan, because doing it all at once seems
easier than doing it piecemeal, and it makes testing this code easier,
too.  CustomScan is excluded because making that work requires a
larger rework of that facility.

Amit Kapila, reviewed and slightly revised by me.