Re: POC: make mxidoff 64 bits

After a bit of thought, I've realized that to be conservative here is the
way to go.

We can reuse a maximum of existing logic. I mean, we can remove offset
wraparound "error logic" and reuse "warning logic". But set the threshold
for "warning logic" to a much higher value. For now, I choose 2^32-1. In
other world, legit logic, in my view, here would be to trigger autovacuum
if the number of offsets (i.e. difference nextOffset - oldestOffset)
exceeds 2^32-1. PFA patch set.

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Best regards,
Maxim Orlov.