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/planner.c

@@ -196,6 +196,7 @@ standard_planner(Query *parse, int cursorOptions, ParamListInfo boundParams)
 	glob->nParamExec = 0;
 	glob->lastPHId = 0;
 	glob->lastRowMarkId = 0;
+	glob->lastPlanNodeId = 0;
 	glob->transientPlan = false;
 	glob->hasRowSecurity = false;