Enable parallelism for prepared statements and extended query protocol.

Parallel query can't handle running a query only partially rather than
to completion.  However, there seems to be no way to run a statement
prepared via SQL PREPARE other than to completion, so we can enable it
there without a problem.

The situation is more complicated for the extend query protocol.
libpq seems to provide no way to send an Execute message with a
non-zero rowcount, but some other client might.  If that happens, and
a parallel plan was chosen, we'll execute the parallel plan without
using any workers, which may be somewhat inefficient but should still
work.  Hopefully this won't be a problem; users can always set
max_parallel_degree=0 to avoid choosing parallel plans in the first
place.

Amit Kapila, reviewed by me.

src/backend/commands/prepare.c

@@ -159,7 +159,7 @@ PrepareQuery(PrepareStmt *stmt, const char *queryString)
 					   nargs,
 					   NULL,
 					   NULL,
-					   0,		/* default cursor options */
+					   CURSOR_OPT_PARALLEL_OK,	/* allow parallel mode */
 					   true);	/* fixed result */
 
 	/*

src/backend/tcop/postgres.c

@@ -1381,7 +1381,7 @@ exec_parse_message(const char *query_string,	/* string to execute */
 					   numParams,
 					   NULL,
 					   NULL,
-					   0,		/* default cursor options */
+					   CURSOR_OPT_PARALLEL_OK,	/* allow parallel mode */
 					   true);	/* fixed result */
 
 	/* If we got a cancel signal during analysis, quit */