Change cost for REF to take into account cost for 1 extra key read_next

The main difference in code path between EQ_REF and REF is that for
REF we have to do an extra read_next on the index to check that there
is no more matching rows.

Before this patch we added a preference of EQ_REF by ensuring that REF
would always estimate to find at least 2 rows.

This patch adds the cost of the extra key read_next to REF access and
removes the code that limited REF to at least 2 rows. For some queries
this can have a big effect as the total estimated rows will be halved
for each REF table with 1 rows.

multi_range cost calculations are also changed to take into account
the difference between EQ_REF and REF.

The effect of the patch to the test suite:
- About 80 test case changed
- Almost all changes where for EXPLAIN where estimated rows for REF
  where changed from 2 to 1.
- A few test cases using explain extended had a change of 'filtered'.
  This is because of the estimated rows are now closer to the
  calculated selectivity.
- A very few test had a change of table order.
  This is because the change of estimated rows from 2 to 1 or the small
  cost change for REF
  (main.subselect_sj_jcl6, main.group_by, main.dervied_cond_pushdown,
  main.distinct, main.join_nested, main.order_by, main.join_cache)
- No key statistics and the estimated rows are now smaller which cased
  estimated filtering to be lower.
  (main.subselect_sj_mat)
- The number of total rows are halved.
  (main.derived_cond_pushdown)
- Plans with 1 row changed to use RANGE instead of REF.
  (main.group_min_max)
- ALL changed to REF
  (main.key_diff)
- Key changed from ref + index_only to PRIMARY key for InnoDB, as
  OPTIMIZER_ROW_LOOKUP_COST + OPTIMIZER_ROW_NEXT_FIND_COST is smaller than
  OPTIMIZER_KEY_LOOKUP_COST + OPTIMIZER_KEY_NEXT_FIND_COST.
  (main.join_outer_innodb)
- Cost changes printouts
  (main.opt_trace*)
- Result order change
  (innodb_gis.rtree)

mysql-test/main/order_by.result

@@ -3033,17 +3033,17 @@ EXPLAIN
 SELECT t1.a FROM t1 LEFT JOIN t2 ON t1.a=t2.a ORDER BY t1.a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	8	Using index
-1	SIMPLE	t2	ref	i_a	i_a	5	test.t1.a	2	Using index
+1	SIMPLE	t2	ref	i_a	i_a	5	test.t1.a	1	Using index
 EXPLAIN 
 SELECT t1.a FROM t1 LEFT JOIN t2 ON t1.a=t2.a ORDER BY t1.a LIMIT 8;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	4	Using index
-1	SIMPLE	t2	ref	i_a	i_a	5	test.t1.a	2	Using index
+1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	6	Using index
+1	SIMPLE	t2	ref	i_a	i_a	5	test.t1.a	1	Using index
 EXPLAIN 
 SELECT t1.a FROM t1 LEFT JOIN t2 ON t1.a=t2.a ORDER BY t1.a LIMIT 100;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	index	NULL	PRIMARY	4	NULL	8	Using index
-1	SIMPLE	t2	ref	i_a	i_a	5	test.t1.a	2	Using index
+1	SIMPLE	t2	ref	i_a	i_a	5	test.t1.a	1	Using index
 DROP TABLE t1,t2;
 #
 # MDEV-4974 memory leak in 5.5.32-MariaDB-1~wheezy-log