Use streaming I/O in sequential scans.

Instead of calling ReadBuffer() for each block, heap sequential scans
and TID range scans now use the streaming API introduced in b5a9b18cd0.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/flat/CAAKRu_YtXJiYKQvb5JsA2SkwrsizYLugs4sSOZh3EAjKUg%3DgEQ%40mail.gmail.com

src/include/access/heapam.h

@@ -25,6 +25,7 @@
 #include "storage/bufpage.h"
 #include "storage/dsm.h"
 #include "storage/lockdefs.h"
+#include "storage/read_stream.h"
 #include "storage/shm_toc.h"
 #include "utils/relcache.h"
 #include "utils/snapshot.h"
@@ -70,6 +71,20 @@ typedef struct HeapScanDescData
 
 	HeapTupleData rs_ctup;		/* current tuple in scan, if any */
 
+	/* For scans that stream reads */
+	ReadStream *rs_read_stream;
+
+	/*
+	 * For sequential scans and TID range scans to stream reads. The read
+	 * stream is allocated at the beginning of the scan and reset on rescan or
+	 * when the scan direction changes. The scan direction is saved each time
+	 * a new page is requested. If the scan direction changes from one page to
+	 * the next, the read stream releases all previously pinned buffers and
+	 * resets the prefetch block.
+	 */
+	ScanDirection rs_dir;
+	BlockNumber rs_prefetch_block;
+
 	/*
 	 * For parallel scans to store page allocation data.  NULL when not
 	 * performing a parallel scan.