Autovacuum freeze scores wrap around for just-vacuumed tables

Hi,

relation_needs_vacanalyze() computes the freeze score ages as
recentXid - relfrozenxid and recentMulti - relminmxid, but both are
read before pg_class is scanned. If a table's relfrozenxid or
relminmxid advances in between, for example by a concurrent VACUUM or
a table rewrite, the subtraction wraps to about 4 billion and the
just-vacuumed table gets a score around 1e44, which can put it at the
front of autovacuum's list. The window grows with the number of
relations; with a few thousand tables, pg_stat_autovacuum_scores shows
it hundreds of times a minute.

0001 treats the age as zero in that case. 0002 is a TAP test, using
the existing autovacuum-worker-start injection point, that shows it for
an autovacuum worker; it's for the record rather than for commit.

Please find the attached patches. Thoughts?

Regards,
Baji Shaik