XLOG stuff for sequences.

src/backend/access/transam/rmgr.c

src/backend/access/transam/xact.c

src/backend/access/transam/xlog.c

src/backend/commands/sequence.c

src/backend/utils/misc/guc.c

src/include/access/htup.h

src/include/access/rmgr.h

src/include/access/xlog.h

src/include/catalog/catversion.h

src/include/commands/sequence.h