Change behavior of (m)xid_age

Currently, xid_age() returns INT_MAX for a permanent xid. The comment in 
the function that 'Permanent XIDs are always infinitely old' may be 
technically correct, but returning INT_MAX is a useless behavior because 
it actually makes it look like that XID is in immediate wraparound 
danger. I think we should change it to return either 0, -1, or INT_MIN. 
To me, 0 makes the most sense for monitoring relfrozenxid.
-- 
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com