br restore is very slow when there are many metafiles

[Leavrth]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. snapshot restore

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

1. success

3. What did you see instead (Required)

1. download and parse the metafiles slowly

4. What is your TiDB version? (Required)

master