Speed up planner's scanning for parallel-query hazards.

We need to scan the whole parse tree for parallel-unsafe functions.
If there are none, we'll later need to determine whether particular
subtrees contain any parallel-restricted functions.  The previous coding
retained no knowledge from the first scan, even though this is very
wasteful in the common case where the query contains only parallel-safe
functions.  We can bypass all of the later scans by remembering that fact.
This provides a small but measurable speed improvement when the case
applies, and shouldn't cost anything when it doesn't.

Patch by me, reviewed by Robert Haas

Discussion: <3740.1471538387@sss.pgh.pa.us>

src/backend/nodes/outfuncs.c

@@ -2029,6 +2029,7 @@ _outPlannerGlobal(StringInfo str, const PlannerGlobal *node)
 	WRITE_BOOL_FIELD(dependsOnRole);
 	WRITE_BOOL_FIELD(parallelModeOK);
 	WRITE_BOOL_FIELD(parallelModeNeeded);
+	WRITE_CHAR_FIELD(maxParallelHazard);
 }
 
 static void