执行计划中执行信息distsql_concurrency应该展示曾经用到的最大并发度

## Enhancement

如下执行计划，在执行计划中tidb_distsql_scan_concurrency=min(cop_tasks,tidb_distsql_scan_concurrency)，但这只是总的可以使用的队列深度，如果可以提供语句执行过程中曾用到的最大并发度那么可以判断语句慢在前端应用取数还是后端tikv供数方便调整tidb_distsql_scan_concurrency参数来最佳效率的同时尽量减少内存的使用。
```

mysql> explain analyze select * from tpch.customer;
+-----------------------+------------+---------+-----------+----------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------+------+
| id | estRows | actRows | task | access object | execution info | operator info | memory | disk |
+-----------------------+------------+---------+-----------+----------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------+------+
| TableReader_5 | 1500000.00 | 1500000 | root | | time:1.84s, loops:1468, cop_task: {num: 4, max: 1.84s, min: 1.35s, avg: 1.54s, p95: 1.84s, max_proc_keys: 524614, p95_proc_keys: 524614, tot_proc: 3.81s, tot_wait: 7ms, rpc_num: 4, rpc_time: 6.14s, copr_cache: disabled, distsql_concurrency: 15} | data:TableFullScan_4 | 201.5 MB | N/A |
| └─TableFullScan_4 | 1500000.00 | 1500000 | cop[tikv] | table:customer | tikv_task:{proc max:874ms, min:346ms, avg: 587.8ms, p80:874ms, p95:874ms, iters:1483, tasks:4}, scan_detail: {total_process_keys: 1500000, total_process_keys_size: 305225771, total_keys: 1731124, get_snapshot_time: 8.42ms, rocksdb: {delete_skipped_count: 1268880, key_skipped_count: 1731120, block: {cache_hit_count: 22, read_count: 5631, read_byte: 145.8 MB, read_time: 176.7ms}}} | keep order:false | N/A | N/A |
+-----------------------+------------+---------+-----------+----------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------+----------+------+
2 rows in set (1.85 sec)
```