Perform distributed planning in the calling memory context

Previously we used, for historical reasons, MessageContext.
That is problematic if a single message from the client
causes a lot of statements to be planned. E.g. for the
copy_to_distributed_table script one insert statement
is planned for each row inserted via COPY, and only freed
when COPY has finished.

src/backend/distributed/planner/multi_planner.c

@@ -52,19 +52,10 @@ multi_planner(Query *parse, int cursorOptions, ParamListInfo boundParams)
 
 	if (NeedsDistributedPlanning(parse))
 	{
-		MemoryContext oldcontext = NULL;
+		MultiPlan *physicalPlan = CreatePhysicalPlan(parse);
-		MultiPlan *physicalPlan = NULL;
-
-		/* Switch to top level message context */
-		oldcontext = MemoryContextSwitchTo(MessageContext);
-
-		physicalPlan = CreatePhysicalPlan(parse);
 
 		/* store required data into the planned statement */
 		result = MultiQueryContainerNode(result, physicalPlan);
-
-		/* Now switch back to original context */
-		MemoryContextSwitchTo(oldcontext);
 	}
 
 	return result;