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

[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.

lp bug: https://launchpad.net/bugs/1177965 fixed

bzr revid: odo@openerp.com-20130620141114-6q7g28jart3zceoi