Lmgr cleanup, new locking modes for LLL.

src/backend/storage/buffer/bufmgr.c

src/backend/storage/lmgr/lmgr.c

src/backend/storage/lmgr/multi.c

src/backend/utils/cache/relcache.c

src/include/storage/buf_internals.h

src/include/storage/lmgr.h

src/include/storage/lock.h

src/include/storage/multilev.h