Use streaming read I/O in BRIN vacuum scan.

This commit implements streaming read I/O for BRIN vacuum
scans. Although BRIN indexes tend to be relatively small by design,
performance tests have shown performance improvements.

Author: Arseniy Mukhin <arseniy.mukhin.dev@gmail.com>
Discussion: https://postgr.es/m/CAE7r3ML01aiq9Th_1OSz7U7Aq2pWbhMLoz5T%2BPXcg8J9ZAPFFA%40mail.gmail.com

src/backend/access/brin/brin.c

@@ -2171,28 +2171,42 @@ union_tuples(BrinDesc *bdesc, BrinMemTuple *a, BrinTuple *b)
 static void
 brin_vacuum_scan(Relation idxrel, BufferAccessStrategy strategy)
 {
-	BlockNumber nblocks;
-	BlockNumber blkno;
+	BlockRangeReadStreamPrivate p;
+	ReadStream *stream;
+	Buffer		buf;
+
+	p.current_blocknum = 0;
+	p.last_exclusive = RelationGetNumberOfBlocks(idxrel);
+
+	/*
+	 * It is safe to use batchmode as block_range_read_stream_cb takes no
+	 * locks.
+	 */
+	stream = read_stream_begin_relation(READ_STREAM_MAINTENANCE |
+										READ_STREAM_FULL |
+										READ_STREAM_USE_BATCHING,
+										strategy,
+										idxrel,
+										MAIN_FORKNUM,
+										block_range_read_stream_cb,
+										&p,
+										0);
 
 	/*
 	 * Scan the index in physical order, and clean up any possible mess in
 	 * each page.
 	 */
-	nblocks = RelationGetNumberOfBlocks(idxrel);
-	for (blkno = 0; blkno < nblocks; blkno++)
+	while ((buf = read_stream_next_buffer(stream, NULL)) != InvalidBuffer)
 	{
-		Buffer		buf;
-
 		CHECK_FOR_INTERRUPTS();
 
-		buf = ReadBufferExtended(idxrel, MAIN_FORKNUM, blkno,
-								 RBM_NORMAL, strategy);
-
 		brin_page_cleanup(idxrel, buf);
 
 		ReleaseBuffer(buf);
 	}
 
+	read_stream_end(stream);
+
 	/*
 	 * Update all upper pages in the index's FSM, as well.  This ensures not
 	 * only that we propagate leaf-page FSM updates made by brin_page_cleanup,