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/subselect2.result

@@ -162,7 +162,7 @@ EXPLAIN
 SELECT * FROM t2,t3 WHERE (2,9) IN (SELECT DISTINCT a,pk FROM t1) OR a = b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t2	index	a	a	5	NULL	2	Using where; Using index
-1	PRIMARY	t3	ref	b	b	5	test.t2.a	2	Using index
+1	PRIMARY	t3	ref	b	b	5	test.t2.a	1	Using index
 2	SUBQUERY	t1	const	PRIMARY,a	a	9	const,const	1	Using where; Using index
 SELECT * FROM t2,t3 WHERE (2,9) IN (SELECT DISTINCT a,pk FROM t1) OR a = b;
 pk	a	b
@@ -171,7 +171,7 @@ EXPLAIN
 SELECT * FROM t2,t3 WHERE (2,9) IN (SELECT DISTINCT a,pk FROM v1) OR a = b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t2	index	a	a	5	NULL	2	Using where; Using index
-1	PRIMARY	t3	ref	b	b	5	test.t2.a	2	Using index
+1	PRIMARY	t3	ref	b	b	5	test.t2.a	1	Using index
 2	SUBQUERY	t1	const	PRIMARY,a	a	9	const,const	1	Using where; Using index
 SELECT * FROM t2,t3 WHERE (2,9) IN (SELECT DISTINCT a,pk FROM v1) OR a = b;
 pk	a	b
@@ -339,7 +339,7 @@ where t1.a = t2.a and ( t1.a = ( select min(a) from t1 ) or 0 );
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	100.00	Using where
 1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	3	100.00	Using where; Using join buffer (flat, BNL join)
-1	PRIMARY	t3	ref	idx	idx	6	func	2	100.00	Using where; Using index
+1	PRIMARY	t3	ref	idx	idx	6	func	1	100.00	Using where; Using index
 2	SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	2	100.00	
 Warnings:
 Note	1003	/* select#1 */ select `test`.`t1`.`a` AS `a`,`test`.`t2`.`a` AS `a`,`test`.`t3`.`a` AS `a` from `test`.`t1` join `test`.`t2` left join `test`.`t3` on(`test`.`t3`.`a` = `test`.`t1`.`a`) where `test`.`t1`.`a` = (/* select#2 */ select min(`test`.`t1`.`a`) from `test`.`t1`) and `test`.`t2`.`a` = (/* select#2 */ select min(`test`.`t1`.`a`) from `test`.`t1`)