Aggressively freeze tables when CLUSTER or VACUUM FULL rewrites them.

We haven't wanted to do this in the past on the grounds that in rare
cases the original xmin value will be needed for forensic purposes, but
commit 37484ad2aacef5ec794f4dd3d5cf814475180a78 removes that objection,
so now we can.

Per extensive discussion, among many people, on pgsql-hackers.