Restore Parameter --tikv-max-restore-concurrency Should Be Limited

## Enhancement

When running BR restore operations, the `--tikv-max-restore-concurrency` parameter defaults to a very large value if tikv increase the config of `import.num-threads`. This can lead to an excessive number of requests being sent to PD,

The original assumption that this parameter is based on the default import.num-threads no longer holds when changes are made, leading to potential performance degradation and instability.

Expected Behavior

The restore operation should work efficiently without overwhelming PD, even when import.num-threads is increased. The default value of `--tikv-max-restore-concurrency` should consider the total number of TiKV nodes, make sure the total request pool in a limited size.