Add xid exhaustion data to statistics gathered

[keithf4]

Since bloat/xid can often be related since bloated tables aren't getting vacuumed as they should, see about adding xid age stats to the data involved.

Databases w/ oldest txid's

SELECT datname, age(datfrozenxid), current_setting('autovacuum_freeze_max_age') FROM pg_database ORDER BY 2 DESC;

Top tables with oldest txid

select oid::regclass, age(relfrozenxid), pg_size_pretty(pg_total_relation_size(oid)) from pg_class where relkind in ('r', 't', 'm', 'p') order by 2 desc limit 100;