[FIX] OPW 592482: browse_record should not prefetch binary fields

This was apparently a long-standing issue due to a
strange handling of the _prefetch attribute on
columns: accessing a column would only trigger
the prefetching if its _prefetch attribute was
True, but the prefetching itself would also
prefetch columns that had _prefetch False.

We clearly want it the other way around, or
at least we want _prefetch to decide whether
a column is included in any given prefetching
pass. We can skip the prefetching pass when
the only field being accessed has _prefetch
False because it is likely the other fields
have already been prefetched separately.
This last subtlety should not make any
noticeable performance difference.
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