some limit can merged with the adjacent topN

[AilinKid]

Enhancement

drop table if exists t;
create table t(a int primary key, b int);
insert into t values (1, 11), (4, 44), (2, 22), (3, 33);
set session tidb_executor_concurrency = 4;
set @@session.tidb_hash_join_concurrency = 5;
set @@session.tidb_distsql_scan_concurrency = 15;

explain select b from (select b from t order by b limit 10 offset 10) as t1 order by b limit 10 offset 5;
mysql> explain select b from (select b from t order by b limit 10 offset 10) as t1 order by b limit 10 offset 5;
+------------------------------+---------+-----------+---------------+--------------------------------+
| id                           | estRows | task      | access object | operator info                  |
+------------------------------+---------+-----------+---------------+--------------------------------+
| Limit_17                     | 4.00    | root      |               | offset:5, count:10             |
| └─TopN_33                    | 4.00    | root      |               | test.t.b, offset:10, count:10  |
|   └─TableReader_39           | 4.00    | root      |               | data:TopN_38                   |
|     └─TopN_38                | 4.00    | cop[tikv] |               | test.t.b, offset:0, count:20   |
|       └─TableFullScan_23     | 4.00    | cop[tikv] | table:t       | keep order:false, stats:pseudo |
+------------------------------+---------+-----------+---------------+--------------------------------+
5 rows in set (0.00 sec)

the top limit can be merged with the adjacent topN as `topN:offset(15):count(5)`

TopN_14	5.00	root		planner__cascades__integration.t.b, offset:15, count:5
└─TableReader_16	20.00	root		data:TopN_17
  └─TopN_17	20.00	cop[tikv]		planner__cascades__integration.t.b, offset:0, count:20
    └─TableFullScan_19	10000.00	cop[tikv]	table:t	keep order:false, stats:pseudo