It's probably an academic concern, but what happens if a backend saves
off cachedFetchXidStatus and then sleeps for a very long time. During
that time an xid wraparound happens and the backend wakes up and
happens to read another unhinted tuple with the same xid and a
different commit status. This is obviously incredibly unlikely, but
shouldn't cachedFetchXid be cleared at some appropriate point --
perhaps end of transaction?
merlin
