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.result

@@ -281,7 +281,7 @@ INSERT INTO t1 VALUES
 (33,5),(33,5),(33,5),(33,5),(34,5),(35,5);
 EXPLAIN SELECT * FROM t1 WHERE a IN(1,2) AND b=5;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	range|filter	a,b	a|b	5|5	NULL	2 (41%)	Using index condition; Using where; Using rowid filter
+1	SIMPLE	t1	range	a,b	a	5	NULL	2	Using index condition; Using where
 SELECT * FROM t1 WHERE a IN(1,2) AND b=5;
 a	b
 DROP TABLE t1;
@@ -2563,7 +2563,7 @@ insert into t2 values
 explain select * from t1,t2
 where a = d and (a,e) in ((3,3),(7,7),(2,2));
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t2	range|filter	idx1,idx2	idx1|idx2	5|5	NULL	3 (60%)	Using index condition; Using where; Using rowid filter
+1	SIMPLE	t2	range	idx1,idx2	idx1	5	NULL	3	Using index condition; Using where
 1	SIMPLE	t1	ref	idx	idx	5	test.t2.d	8	
 explain format=json select * from t1,t2
 where a = d and (a,e) in ((3,3),(7,7),(2,2));
@@ -2581,14 +2581,6 @@ EXPLAIN
           "key": "idx1",
           "key_length": "5",
           "used_key_parts": ["d"],
-          "rowid_filter": {
-            "range": {
-              "key": "idx2",
-              "used_key_parts": ["e"]
-            },
-            "rows": 12,
-            "selectivity_pct": 60
-          },
           "loops": 1,
           "rows": 3,
           "cost": "COST_REPLACED",