Have the rewriteheap code freeze old tuples. This is safe because it is only

Have the rewriteheap code freeze old tuples.  This is safe because it is only
applied to live tuples older than a recent Xmin, not to tuples that may be part
of an update chain.  Those still keep their original markings.

This patch makes it possible for CLUSTER to advance relfrozenxid, thus avoiding
the need of vacuuming the table for Xid wraparound purposes.  That will be
patched separately.

Patch from Heikki Linnakangas.