BUG#628785: multi_range_read.cc:430: int DsMrr_impl::dsmrr_init(): Assertion `do_sort_keys || do_rowid_fetch' failed 
- Make Ds_MrrImpl::check_cpk_scan() follow the execution code' logic: don't
  do MRR scans on clustered PK when mrr_sort_keys=off.