Allow parallelism in COPY (query) TO ...;

Previously this was not allowed, as copy.c didn't set the
CURSOR_OPT_PARALLEL_OK flag when planning the query. Set it.

While the lack of parallel query for COPY isn't strictly speaking a
bug, it does prevent parallelism from being used in a facility
commonly used to run long running queries. Thus backpatch to 9.6.

Author: Andres Freund
Discussion: https://postgr.es/m/20170531231958.ihanapplorptykzm@alap3.anarazel.de
Backpatch: 9.6, where parallelism was introduced.

src/backend/commands/copy.c

@@ -1527,7 +1527,7 @@ BeginCopy(ParseState *pstate,
 		}
 
 		/* plan the query */
-		plan = pg_plan_query(query, 0, NULL);
+		plan = pg_plan_query(query, CURSOR_OPT_PARALLEL_OK, NULL);
 
 		/*
 		 * With row level security and a user using "COPY relation TO", we