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/group_by.result

@@ -552,12 +552,12 @@ a	b
 3	1
 explain select t1.a,t2.b from t1,t2 where t1.a=t2.a group by t1.a,t2.b;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t2	ALL	a	NULL	NULL	NULL	4	Using temporary; Using filesort
-1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	6	Using where; Using join buffer (flat, BNL join)
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	6	Using temporary; Using filesort
+1	SIMPLE	t2	ref	a	a	4	test.t1.a	1	
 explain select t1.a,t2.b from t1,t2 where t1.a=t2.a group by t1.a,t2.b ORDER BY NULL;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t2	ALL	a	NULL	NULL	NULL	4	Using temporary
-1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	6	Using where; Using join buffer (flat, BNL join)
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	6	Using temporary
+1	SIMPLE	t2	ref	a	a	4	test.t1.a	1	
 drop table t1,t2;
 SET @@optimizer_use_condition_selectivity=@save_optimizer_use_condition_selectivity,@@optimizer_switch=@save_optimizer_switch;
 create table t1 (a int, b int);
@@ -1972,8 +1972,8 @@ SELECT a, AVG(t1.b),
 FROM t1 GROUP BY a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	index	NULL	a	10	NULL	9	Using index
-3	DEPENDENT SUBQUERY	t12	ref	a	a	10	func,func	2	Using index condition
-2	DEPENDENT SUBQUERY	t11	ref	a	a	10	func,func	2	Using index condition
+3	DEPENDENT SUBQUERY	t12	ref	a	a	10	func,func	1	Using index condition
+2	DEPENDENT SUBQUERY	t11	ref	a	a	10	func,func	1	Using index condition
 SELECT a, AVG(t1.b),
 (SELECT t11.c FROM t1 t11 WHERE t11.a = t1.a AND t11.b = AVG(t1.b)) AS t11c,
 (SELECT t12.c FROM t1 t12 WHERE t12.a = t1.a AND t12.b = AVG(t1.b)) AS t12c