Fix per-relation memory leakage in autovacuum.

Fix per-relation memory leakage in autovacuum.

PgStat_StatTabEntry and AutoVacOpts structs were leaked until
the end of the autovacuum worker's run, which is bad news if
there are a lot of relations in the database.

Note: pfree'ing the PgStat_StatTabEntry structs here seems a bit
risky, because pgstat_fetch_stat_tabentry_ext does not guarantee
anything about whether its result is long-lived.  It appears okay
so long as autovacuum forces PGSTAT_FETCH_CONSISTENCY_NONE, but
I think that API could use a re-think.

Also ensure that the VacuumRelation structure passed to
vacuum() is in recoverable storage.

Back-patch to v15 where we started to manage table statistics
this way.  (The AutoVacOpts leakage is probably older, but
I'm not excited enough to worry about just that part.)

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/285483.1746756246@sss.pgh.pa.us
Backpatch-through: 15