Parallel executor support.

This code provides infrastructure for a parallel leader to start up
parallel workers to execute subtrees of the plan tree being executed
in the master.  User-supplied parameters from ParamListInfo are passed
down, but PARAM_EXEC parameters are not.  Various other constructs,
such as initplans, subplans, and CTEs, are also not currently shared.
Nevertheless, there's enough here to support a basic implementation of
parallel query, and we can lift some of the current restrictions as
needed.

Amit Kapila and Robert Haas

src/backend/nodes/outfuncs.c

@@ -271,6 +271,7 @@ _outPlanInfo(StringInfo str, const Plan *node)
 	WRITE_FLOAT_FIELD(total_cost, "%.2f");
 	WRITE_FLOAT_FIELD(plan_rows, "%.0f");
 	WRITE_INT_FIELD(plan_width);
+	WRITE_INT_FIELD(plan_node_id);
 	WRITE_NODE_FIELD(targetlist);
 	WRITE_NODE_FIELD(qual);
 	WRITE_NODE_FIELD(lefttree);