- In Spider, calling cmp_ref() can be very expensive. In ha_partition.cc
  we don't anymore sort rows according to position for the Spider
  engine.
- Removed Spider specific call info(HA_EXTRA_STARTING_ORDERED_INDEX_SCAN)
  from handle_ordered_index_scan(). It's caused performance issues and
  does not change results for queries with ORDER BY.
- The visible effect of this patch is that for some storage engines,
  rows may be returned in a different order if there is no ORDER BY clause.

- Based in Spiral Patch 052:
  052_mariadb-10.2.0.add_partition_skip_pk_sort_for_non_clustered_index
  MDEV-7748
- The major difference from original patch is that there is no variable to
  get the old behaviour.

Other things:
- Optimized ha_partition::cmp_ref() and cmp_part_ids() to make them
  simpler and faster.
- Changed arguments to cmp_key_part_id() to be same as
  cmp_key_rowid_part_id to simplify code.

Original author: Kentoku SHIBA
First reviewer:  Jacob Mathew
Second reviewer: Michael Widenius