Add maintenance_io_concurrency flag to some read stream users

Index vacuuming and [auto]prewarm AIO concurrency should be governed by
maintenance_io_concurrency. As such, pass those read stream users the
READ_STREAM_MAINTENANCE flag which will calculate their read stream
distance with maintenance_io_concurrency instead of
effective_io_concurrency. This was an oversight in the original commits
making those operations use the read stream API.

Discussion: https://postgr.es/m/flat/CAAKRu_aopDxTo4b41Mt_7Zc-z0_ngocrY8SFCCY6Aph1HgwuNw%40mail.gmail.com

contrib/pg_prewarm/autoprewarm.c

@@ -621,7 +621,8 @@ autoprewarm_database_main(Datum main_arg)
 					.nblocks = nblocks,
 			};
 
-			stream = read_stream_begin_relation(READ_STREAM_DEFAULT |
+			stream = read_stream_begin_relation(READ_STREAM_MAINTENANCE |
+												READ_STREAM_DEFAULT |
 												READ_STREAM_USE_BATCHING,
 												NULL,
 												rel,

contrib/pg_prewarm/pg_prewarm.c

@@ -202,7 +202,8 @@ pg_prewarm(PG_FUNCTION_ARGS)
 		 * It is safe to use batchmode as block_range_read_stream_cb takes no
 		 * locks.
 		 */
-		stream = read_stream_begin_relation(READ_STREAM_FULL |
+		stream = read_stream_begin_relation(READ_STREAM_MAINTENANCE |
+											READ_STREAM_FULL |
 											READ_STREAM_USE_BATCHING,
 											NULL,
 											rel,

src/backend/access/gist/gistvacuum.c

@@ -215,7 +215,8 @@ gistvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
 	 * It is safe to use batchmode as block_range_read_stream_cb takes no
 	 * locks.
 	 */
-	stream = read_stream_begin_relation(READ_STREAM_FULL |
+	stream = read_stream_begin_relation(READ_STREAM_MAINTENANCE |
+										READ_STREAM_FULL |
 										READ_STREAM_USE_BATCHING,
 										info->strategy,
 										rel,

src/backend/access/nbtree/nbtree.c

@@ -1232,7 +1232,8 @@ btvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
 	 * It is safe to use batchmode as block_range_read_stream_cb takes no
 	 * locks.
 	 */
-	stream = read_stream_begin_relation(READ_STREAM_FULL |
+	stream = read_stream_begin_relation(READ_STREAM_MAINTENANCE |
+										READ_STREAM_FULL |
 										READ_STREAM_USE_BATCHING,
 										info->strategy,
 										rel,

src/backend/access/spgist/spgvacuum.c

@@ -827,7 +827,8 @@ spgvacuumscan(spgBulkDeleteState *bds)
 	 * It is safe to use batchmode as block_range_read_stream_cb takes no
 	 * locks.
 	 */
-	stream = read_stream_begin_relation(READ_STREAM_FULL |
+	stream = read_stream_begin_relation(READ_STREAM_MAINTENANCE |
+										READ_STREAM_FULL |
 										READ_STREAM_USE_BATCHING,
 										bds->info->strategy,
 										index,