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/optimizer/plan/setrefs.c

@@ -174,6 +174,8 @@ static bool extract_query_dependencies_walker(Node *node,
  * Currently, relations and user-defined functions are the only types of
  * objects that are explicitly tracked this way.
  *
+ * 7. We assign every plan node in the tree a unique ID.
+ *
  * We also perform one final optimization step, which is to delete
  * SubqueryScan plan nodes that aren't doing anything useful (ie, have
  * no qual and a no-op targetlist).  The reason for doing this last is that
@@ -436,6 +438,9 @@ set_plan_refs(PlannerInfo *root, Plan *plan, int rtoffset)
 	if (plan == NULL)
 		return NULL;
 
+	/* Assign this node a unique ID. */
+	plan->plan_node_id = root->glob->lastPlanNodeId++;
+
 	/*
 	 * Plan-type-specific fixes
 	 */