Br restore download SST throughput not stable throughout the whole restore process

[fubinzh]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. Run br restore to restore a cluster of 110TB

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

Download SST throughput should be use as much of the physical bandwidth as possible, and should be stable during the whole process.

3. What did you see instead (Required)

Download SST throughput can reach the disk throughtput limit (1200MB/s) sometime, but it is not stable enough, there are some downgrade to 1GB or lower.

4. What is your TiDB version? (Required)

v7.6.0

[fubinzh]

/component br
/severity moderate

[3pointer]

The issue is suspect it reach the grpc server limitaion: grpc-concurrent-stream which means handling the requests throughput in a stream. and the default value is 1024.