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/include/access/commit_ts.h

@@ -14,6 +14,7 @@
 #include "access/xlog.h"
 #include "datatype/timestamp.h"
 #include "replication/origin.h"
+#include "storage/sync.h"
 #include "utils/guc.h"
 
 
@@ -45,6 +46,8 @@ extern void SetCommitTsLimit(TransactionId oldestXact,
 							 TransactionId newestXact);
 extern void AdvanceOldestCommitTsXid(TransactionId oldestXact);
 
+extern int	committssyncfiletag(const FileTag *ftag, char *path);
+
 /* XLOG stuff */
 #define COMMIT_TS_ZEROPAGE		0x00
 #define COMMIT_TS_TRUNCATE		0x10