tidb_merge_partition_stats_concurrency cannot take effect immediately

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

when you have a analyze task in the merge global stats period. we want to change the tidb_merge_partition_stats_concurrency.

### 2. What did you expect to see? (Required)

it can take effect immediately.

### 3. What did you see instead (Required)

it cannot take effect.

### 4. What is your TiDB version? (Required)