optimizer-adjust_secondary_key_costs is added to provide 2 small
adjustments to the 10.x optimizer cost model. This can be used in the
case where the optimizer wrongly uses a secondary key instead of a
clustered primary key.

The reason behind this change is that MariaDB 10.x does not take into
account that for engines like InnoDB, that scanning a primary key can be
up to 7x faster than scanning a secondary key + read the row data trough
the primary key.

The different values for optimizer_adjust_secondary_key_costs are:

optimizer_adjust_secondary_key_costs=0
- No changes to current model

optimizer_adjust_secondary_key_costs=1
- Ensure that the cost of of secondary indexes has a cost of at
  least 5x times the cost of a clustered primary key (if one exists).
  This disables part of the worst_seek optimization described below.

optimizer_adjust_secondary_key_costs=2
- Disable "worst_seek optimization" and adjust filter cost slightly
  (add cost of 1 if filter is used).

The idea behind 'worst_seek optimization' is that we limit the
cost for all non clustered ref access to the least of:
- best-rows-by-range (or all rows in no range found) / 10
- scan-time-table (roughly number of file blocks to scan table) * 3

In addition we also do not try to use rowid_filter if number of rows
estimated for 'ref' access is less than the worst_seek limitation.

The idea is that worst_seek is trying to take into account that if
we do a lot of accesses through a key, this is likely to be cached.
However it only does this for secondary keys, and not for clustered
keys or index only reads.

The effect of the worst_seek are:
- In some cases 'ref' will have a much lower cost than range or using
  a clustered key.
- Some possible rowid filters for secondary keys will be ignored.

When implementing optimizer_adjust_secondary_key_costs=2, I noticed
that there is a slightly different costs for how ref+filter and
range+filter are calculated.  This caused a lot of range and
range+filter to change to ref+filter, which is not good as
range+filter provides the optimizer a better estimate of how many
accepted rows there will be in the result set.
Adding a extra small cost (1 seek) when using filter mitigated the
above problems in almost all cases.

This patch should not be applied to MariaDB 11.0 as worst_seeks is
removed in 11.0 and the cost calculation for clustered keys, secondary
keys, index scan and filter is more exact.

Test case changes for --optimizer-adjust_secondary_key_costs=1
(Fix secondary key costs to be 5x of primary key):

- stat_tables_innodb:
  - Complex change (probably ok as number of rows are really small)
    - ref over 1 row changed to range over 10 rows with join buffer
    - ref over 5 rows changed to eq_ref
    - secondary ref over 1 row changed to ref of primary key over 4 rows
    - Change of key to use longer key with index pushdown (a little
      bit worse but not significant).
  - Change to use secondary (1 row) -> primary (4 rows)
- rowid_filter_innodb:
  - index_merge (2 rows) & ref (1) -> all (23 rows) -> primary eq_ref.

Test case changes for --optimizer-adjust_secondary_key_costs=2
(remove of worst_seeks & adjust filter cost):

- stat_tables_innodb:
  - Join order change (probably ok as number of rows are really small)
  - ref (5 rows) & ref(1 row) changed to range (10 rows & join buffer)
    & eq_ref.
- selectivity_innodb:
  - ref -> ref|filter  (ok)
- rowid_filter_innodb:
  - ref -> ref|filter (ok)
  - range|filter (64 rows) changed to ref|filter (128 rows).
    ok as ref|filter outputs wrong number of rows in explain.
- range, range_mrr_icp:
  -ref (500 rows -> ALL (1000 rows) (ok)
- select_pkeycache, select, select_jcl6:
  - ref|filter (2 rows) -> ref (2 rows) (ok)
- selectivity:
  - ref -> ref_filter (ok)
- range:
  - Change of 'filtered' but no stat or plan change (ok)
- selectivity:
 - ref -> ref+filter (ok)
 - Change of filtered but no plan change (ok)
- join_nested_jcl6:
  - range -> ref|filter (ok as only 2 rows)
- subselect3, subselect3_jcl6:
  - ref_or_null (4 rows) -> ALL (10 rows) (ok)
  - Index_subquery (4 rows) -> ALL (10 rows)  (ok)
- partition_mrr_myisam, partition_mrr_aria and partition_mrr_innodb:
  - Uses ALL instead of REF for a key value that is the same for > 50%
    of rows.  (good)
order_by_innodb:
  - range (200 rows) -> ref (20 rows)+filesort (ok)
- subselect_sj2_mat:
  - One test changed. One ALL removed and replaced with eq_ref. Likely
    to be better.
- join_cache:
  - Changed ref over 60% of the rows to use hash join (ok)
- opt_tvc:
  - Changed to use eq_ref instead of ref with plan change (probably ok)
- opt_trace:
  - No worst/max seeks clipping (good).
  - Almost double range_scan_time and index_scan_time (ok).
- rowid_filter:
  - ref -> ref|filtered (ok)
  - range|filter (77 rows) changed to ref|filter (151 rows).  Proably
    ok as ref|filter outputs wrong number of rows in explain.

Reviewer: Sergei Petrunia <sergey@mariadb.com>