Enable use of Rowid Filter optimization with eq_ref access.
Use the following assumptions:
- Assume index-only access cost is 50% of non-index-only access cost.
- Take into account that "Eq_ref access cache" reduces the number of
  lookups eq_ref access will make.
  = This means the number of Rowid Filter checks is reduced also
  = Eq_ref access cost is computed using that assumption (see
    prev_record_reads() call), so we should use it in all cost '
    computations.