Fix LMGR for MVCC.

src/backend/access/heap/hio.c

src/backend/storage/lmgr/lmgr.c

src/backend/storage/lmgr/lock.c

src/backend/storage/lmgr/proc.c

src/include/storage/lmgr.h

src/include/storage/lock.h

src/include/storage/proc.h