Adjust costs for rowid filter

- Use log2() insted of log()
- Added missing ''+' when calculating rowid setup cost
- Adjusted ROWID_FILTER_PER_ELEMENT_MODIFIER (from 3 to 1)

Other things:
- Adjusted cost for index_merge where rows_out < 1.0

The effects of the changes:
- rowid filter will have higher setup cost
- rowid filter will have slightly less costs per row

This can be seen in mtr where some tests, with 'small tables or
that uses rowid filters with many rows, will not use rowid filter anymore.

mysql-test/main/range_innodb.result

@@ -98,7 +98,7 @@ SET @saved_dbug = @@GLOBAL.debug_dbug;
 set @@global.debug_dbug="+d,ha_index_init_fail";
 explain select * from t1 where a=10 and b=10;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	ref|filter	a,b	a|b	5|5	const	50 (2%)	Using where; Using rowid filter
+1	SIMPLE	t1	index_merge	a,b	a,b	5,5	NULL	1	Using intersect(a,b); Using where
 select * from t1 where a=10 and b=10;
 ERROR HY000: Table definition has changed, please retry transaction
 DROP TABLE t0,t1;