Move parallel vacuum code to vacuumparallel.c.

This commit moves parallel vacuum related code to a new file
commands/vacuumparallel.c so that any table AM supporting indexes can
utilize parallel vacuum in order to call index AM callbacks (ambulkdelete
and amvacuumcleanup) with parallel workers.

Another reason for this refactoring is that the parallel vacuum isn't
specific to heap so it doesn't make sense to keep this code in
heap/vacuumlazy.c.

Author: Masahiko Sawada, based on suggestion from Andres Freund
Reviewed-by: Hou Zhijie, Amit Kapila, Haiying Tang
Discussion: https://www.postgresql.org/message-id/20211030212101.ae3qcouatwmy7tbr%40alap3.anarazel.de

Author: Amit Kapila

src/backend/access/heap/vacuumlazy.c

@@ -14,7 +14,6 @@
 
 #include "postgres.h"
 
-#include "access/heapam.h"
 #include "access/nbtree.h"
 #include "access/parallel.h"
 #include "access/session.h"
@@ -25,6 +24,7 @@
 #include "catalog/pg_enum.h"
 #include "catalog/storage.h"
 #include "commands/async.h"
+#include "commands/vacuum.h"
 #include "executor/execParallel.h"
 #include "libpq/libpq.h"
 #include "libpq/pqformat.h"

src/backend/access/transam/parallel.c

@@ -59,6 +59,7 @@ OBJS = \
 	typecmds.o \
 	user.o \
 	vacuum.o \
+	vacuumparallel.o \
 	variable.o \
 	view.o
 

src/backend/commands/Makefile

@@ -7,8 +7,9 @@
  * commands, (b) code to compute various vacuum thresholds, and (c) index
  * vacuum code.
  *
- * VACUUM for heap AM is implemented in vacuumlazy.c, ANALYZE in analyze.c, and
- * VACUUM FULL is a variant of CLUSTER, handled in cluster.c.
+ * VACUUM for heap AM is implemented in vacuumlazy.c, parallel vacuum in
+ * vacuumparallel.c, ANALYZE in analyze.c, and VACUUM FULL is a variant of
+ * CLUSTER, handled in cluster.c.
  *
  *
  * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group