DS-MRR improvements: review feedback

- Switch from one bi-directional buffer class to two 
  virtual inheritance-based forward and backward buffer classes.

mysql-test/r/join_nested_jcl6.result

@@ -865,12 +865,12 @@ LEFT JOIN
 (t1,t2)
 ON t3.a=1 AND t3.b=t2.b AND t2.b=t4.b;
 a	b	a	b	a	b
-4	2	1	2	4	2
 4	2	1	2	3	2
 4	2	1	2	4	2
 4	2	1	2	3	2
 4	2	1	2	4	2
 4	2	1	2	3	2
+4	2	1	2	4	2
 NULL	NULL	2	2	3	2
 NULL	NULL	2	2	4	2
 EXPLAIN EXTENDED
@@ -1105,8 +1105,8 @@ t0.b=t1.b AND
 (t8.b=t9.b OR t8.c IS NULL) AND
 (t9.a=1);
 a	b	a	b	a	b	a	b	a	b	a	b	a	b	a	b	a	b	a	b
-1	2	3	2	4	2	1	2	4	2	2	2	6	2	2	2	0	2	1	2
 1	2	3	2	4	2	1	2	3	2	2	2	6	2	2	2	0	2	1	2
+1	2	3	2	4	2	1	2	4	2	2	2	6	2	2	2	0	2	1	2
 1	2	3	2	4	2	1	2	3	2	3	1	6	2	1	1	NULL	NULL	1	1
 1	2	3	2	4	2	1	2	4	2	3	1	6	2	1	1	NULL	NULL	1	1
 1	2	3	2	4	2	1	2	3	2	3	1	6	2	1	1	NULL	NULL	1	2
@@ -1785,8 +1785,8 @@ ON t7.b=t8.b AND t6.b < 10
 ON t6.b >= 2 AND t5.b=t7.b AND
 (t8.a > 0 OR t8.c IS NULL);
 a	b	a	b	a	b	a	b
-2	2	3	2	2	2	1	2
 2	2	1	2	2	2	1	2
+2	2	3	2	2	2	1	2
 1	1	1	2	1	1	NULL	NULL
 1	1	3	2	1	1	NULL	NULL
 3	3	NULL	NULL	NULL	NULL	NULL	NULL

mysql-test/r/join_outer_jcl6.result

@@ -352,14 +352,14 @@ Thimble Smith	Happy	3	3
 Lilliana Angelovska	NULL	NULL	NULL
 select t1.name, t2.name, t2.id,t3.id from t1 right join t2 on (t1.id = t2.owner) right join t1 as t3 on t3.id=t2.owner;
 name	name	id	id
-Antonio Paz	Perrito	2	1
 Antonio Paz	El Gato	1	1
+Antonio Paz	Perrito	2	1
 Thimble Smith	Happy	3	3
 NULL	NULL	NULL	2
 select t1.name, t2.name, t2.id, t2.owner, t3.id from t1 left join t2 on (t1.id = t2.owner) right join t1 as t3 on t3.id=t2.owner;
 name	name	id	owner	id
-Antonio Paz	Perrito	2	1	1
 Antonio Paz	El Gato	1	1	1
+Antonio Paz	Perrito	2	1	1
 Thimble Smith	Happy	3	3	3
 NULL	NULL	NULL	NULL	2
 drop table t1,t2;
@@ -413,9 +413,9 @@ insert into t2 values (1, 2, 3),(2, 2, 8), (4,3,9),(3,2,10);
 select t1.*, t2.* from t1 left join t2 on t1.n = t2.n and
 t1.m = t2.m where t1.n = 1;
 n	m	o	n	m	o
-1	2	9	1	2	3
-1	2	7	1	2	3
 1	2	11	1	2	3
+1	2	7	1	2	3
+1	2	9	1	2	3
 1	3	9	NULL	NULL	NULL
 select t1.*, t2.* from t1 left join t2 on t1.n = t2.n and
 t1.m = t2.m where t1.n = 1 order by t1.o;