Move logic related to WAL replay of Heap/Heap2 into its own file

This brings more clarity to heapam.c, by cleanly separating all the
logic related to WAL replay and the rest of Heap and Heap2, similarly
to other RMGRs like hash, btree, etc.

The header reorganization is also nice in heapam.c, cutting half of the
headers required.

Author: Li Yong
Reviewed-by: Sutou Kouhei, Michael Paquier
Discussion: https://postgr.es/m/EFE55E65-D7BD-4C6A-B630-91F43FD0771B@ebay.com

Author: michaelpq

src/backend/access/heap/Makefile

@@ -16,6 +16,7 @@ OBJS = \
 	heapam.o \
 	heapam_handler.o \
 	heapam_visibility.o \
+	heapam_xlog.o \
 	heaptoast.o \
 	hio.o \
 	pruneheap.o \