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/join_outer.result

@@ -1230,7 +1230,7 @@ EXPLAIN
 SELECT t1.id, a FROM t1 LEFT JOIN t2 ON t1.id=t2.id WHERE t2.b IS NULL;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	5	
-1	SIMPLE	t2	ref	idx	idx	4	test.t1.id	2	Using where; Not exists
+1	SIMPLE	t2	ref	idx	idx	4	test.t1.id	1	Using where; Not exists
 flush status;
 SELECT t1.id, a FROM t1 LEFT JOIN t2 ON t1.id=t2.id WHERE t2.b IS NULL;
 id	a
@@ -2080,7 +2080,7 @@ WHERE t1.pk BETWEEN 5 AND 6 AND t1.b IS NULL OR t1.b = 5
 ORDER BY t1.b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	ref	idx	idx	4	const	2	100.00	Using where
-1	SIMPLE	t2	ref	c	c	5	test.t1.a	2	100.00	
+1	SIMPLE	t2	ref	c	c	5	test.t1.a	1	100.00	
 Warnings:
 Note	1003	select `test`.`t1`.`b` AS `b`,`test`.`t2`.`c` AS `c`,`test`.`t2`.`d` AS `d` from `test`.`t2` join `test`.`t1` where `test`.`t2`.`c` = `test`.`t1`.`a` and `test`.`t1`.`b` = 5 order by `test`.`t1`.`b`
 SELECT t1.b, t2.c, t2.d FROM t2 JOIN t1 ON t2.c = t1.a
@@ -2097,7 +2097,7 @@ WHERE t1.pk BETWEEN 5 AND 6 AND t1.b IS NULL OR t1.b = 5
 ORDER BY t1.b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 1	SIMPLE	t1	ref	PRIMARY,idx	idx	4	const	2	100.00	Using where
-1	SIMPLE	t2	ref	c	c	5	test.t1.a	2	100.00	
+1	SIMPLE	t2	ref	c	c	5	test.t1.a	1	100.00	
 Warnings:
 Note	1003	select `test`.`t1`.`b` AS `b`,`test`.`t2`.`c` AS `c`,`test`.`t2`.`d` AS `d` from `test`.`t2` join `test`.`t1` where `test`.`t2`.`c` = `test`.`t1`.`a` and `test`.`t1`.`b` = 5 order by `test`.`t1`.`b`
 SELECT t1.b, t2.c, t2.d FROM t2 LEFT JOIN t1 ON t2.c = t1.a