Split xlog.c into xlog.c and xlogrecovery.c.

This moves the functions related to performing WAL recovery into the new
xlogrecovery.c source file, leaving xlog.c responsible for maintaining
the WAL buffers, coordinating the startup and switch from recovery to
normal operations, and other miscellaneous stuff that have always been in
xlog.c.

Reviewed-by: Andres Freund, Kyotaro Horiguchi, Robert Haas
Discussion: https://www.postgresql.org/message-id/a31f27b4-a31d-f976-6217-2b03be646ffa%40iki.fi

src/backend/access/transam/Makefile

@@ -32,6 +32,7 @@ OBJS = \
 	xlogfuncs.o \
 	xloginsert.o \
 	xlogreader.o \
+	xlogrecovery.o \
 	xlogutils.o
 
 include $(top_srcdir)/src/backend/common.mk

src/backend/access/transam/xact.c

@@ -29,6 +29,7 @@
 #include "access/xact.h"
 #include "access/xlog.h"
 #include "access/xloginsert.h"
+#include "access/xlogrecovery.h"
 #include "access/xlogutils.h"
 #include "catalog/index.h"
 #include "catalog/namespace.h"

src/backend/access/transam/xlogfuncs.c

@@ -19,8 +19,8 @@
 #include <unistd.h>
 
 #include "access/htup_details.h"
-#include "access/xlog.h"
 #include "access/xlog_internal.h"
+#include "access/xlogrecovery.h"
 #include "access/xlogutils.h"
 #include "catalog/pg_type.h"
 #include "funcapi.h"

src/backend/access/transam/xlogutils.c

@@ -20,7 +20,7 @@
 #include <unistd.h>
 
 #include "access/timeline.h"
-#include "access/xlog.h"
+#include "access/xlogrecovery.h"
 #include "access/xlog_internal.h"
 #include "access/xlogutils.h"
 #include "miscadmin.h"
@@ -46,8 +46,8 @@ bool		ignore_invalid_pages = false;
  * process you're running in, use RecoveryInProgress() but only after shared
  * memory startup and lock initialization.
  *
- * This is updated from xlog.c, but lives here because it's mostly read by
- * WAL redo functions.
+ * This is updated from xlog.c and xlogrecovery.c, but lives here because
+ * it's mostly read by WAL redo functions.
  */
 bool		InRecovery = false;