Defer flushing of SLRU files.

Previously, we called fsync() after writing out individual pg_xact,
pg_multixact and pg_commit_ts pages due to cache pressure, leading to
regular I/O stalls in user backends and recovery.  Collapse requests for
the same file into a single system call as part of the next checkpoint,
as we already did for relation files, using the infrastructure developed
by commit 3eb77eba.  This can cause a significant improvement to
recovery performance, especially when it's otherwise CPU-bound.

Hoist ProcessSyncRequests() up into CheckPointGuts() to make it clearer
that it applies to all the SLRU mini-buffer-pools as well as the main
buffer pool.  Rearrange things so that data collected in CheckpointStats
includes SLRU activity.

Also remove the Shutdown{CLOG,CommitTS,SUBTRANS,MultiXact}() functions,
because they were redundant after the shutdown checkpoint that
immediately precedes them.  (I'm not sure if they were ever needed, but
they aren't now.)

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> (parts)
Tested-by: Jakub Wartak <Jakub.Wartak@tomtom.com>
Discussion: https://postgr.es/m/CA+hUKGLJ=84YT+NvhkEEDAuUtVHMfQ9i-N7k_o50JmQ6Rpj_OQ@mail.gmail.com

src/backend/storage/lmgr/predicate.c

@@ -821,9 +821,7 @@ SerialInit(void)
 	SerialSlruCtl->PagePrecedes = SerialPagePrecedesLogically;
 	SimpleLruInit(SerialSlruCtl, "Serial",
 				  NUM_SERIAL_BUFFERS, 0, SerialSLRULock, "pg_serial",
-				  LWTRANCHE_SERIAL_BUFFER);
-	/* Override default assumption that writes should be fsync'd */
-	SerialSlruCtl->do_fsync = false;
+				  LWTRANCHE_SERIAL_BUFFER, SYNC_HANDLER_NONE);
 
 	/*
 	 * Create or attach to the SerialControl structure.
@@ -1052,7 +1050,7 @@ CheckPointPredicate(void)
 	SimpleLruTruncate(SerialSlruCtl, tailPage);
 
 	/*
-	 * Flush dirty SLRU pages to disk
+	 * Write dirty SLRU pages to disk
 	 *
 	 * This is not actually necessary from a correctness point of view. We do
 	 * it merely as a debugging aid.
@@ -1061,7 +1059,7 @@ CheckPointPredicate(void)
 	 * before deleting the file in which they sit, which would be completely
 	 * pointless.
 	 */
-	SimpleLruFlush(SerialSlruCtl, true);
+	SimpleLruWriteAll(SerialSlruCtl, true);
 }
 
 /*------------------------------------------------------------------------*/