Move syncscan.c to src/backend/access/common.

Since the tableam.c code needs to make use of the syncscan.c routines
itself, and since other block-oriented AMs might also want to use it one
day, it didn't make sense for it to live under src/backend/access/heap.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA%2BhUKGLCnG%3DNEAByg6bk%2BCT9JZD97Y%3DAxKhh27Su9FeGWOKvDg%40mail.gmail.com

src/backend/access/common/Makefile

@@ -24,6 +24,7 @@ OBJS = \
 	reloptions.o \
 	scankey.o \
 	session.o \
+	syncscan.o \
 	toast_internals.o \
 	tupconvert.o \
 	tupdesc.o

src/backend/access/common/syncscan.c

@@ -1,7 +1,7 @@
 /*-------------------------------------------------------------------------
  *
  * syncscan.c
- *	  heap scan synchronization support
+ *	  scan synchronization support
  *
  * When multiple backends run a sequential scan on the same table, we try
  * to keep them synchronized to reduce the overall I/O needed.  The goal is
@@ -40,13 +40,13 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  src/backend/access/heap/syncscan.c
+ *	  src/backend/access/common/syncscan.c
  *
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
 
-#include "access/heapam.h"
+#include "access/syncscan.h"
 #include "miscadmin.h"
 #include "storage/lwlock.h"
 #include "storage/shmem.h"

src/backend/access/heap/Makefile

@@ -20,7 +20,6 @@ OBJS = \
 	hio.o \
 	pruneheap.o \
 	rewriteheap.o \
-	syncscan.o \
 	vacuumlazy.o \
 	visibilitymap.o
 

src/backend/access/heap/heapam.c

@@ -41,6 +41,7 @@
 #include "access/parallel.h"
 #include "access/relscan.h"
 #include "access/subtrans.h"
+#include "access/syncscan.h"
 #include "access/sysattr.h"
 #include "access/tableam.h"
 #include "access/transam.h"

src/backend/access/heap/heapam_handler.c

@@ -24,6 +24,7 @@
 #include "access/heaptoast.h"
 #include "access/multixact.h"
 #include "access/rewriteheap.h"
+#include "access/syncscan.h"
 #include "access/tableam.h"
 #include "access/tsmapi.h"
 #include "access/xact.h"

src/backend/access/table/tableam.c

@@ -21,7 +21,7 @@
 
 #include <math.h>
 
-#include "access/heapam.h"		/* for ss_* */
+#include "access/syncscan.h"
 #include "access/tableam.h"
 #include "access/xact.h"
 #include "optimizer/plancat.h"