br: rate limiter seems not effectively controlling disk read during snapshot backup

## Bug Report

One of our users is seeing high disk read TP during snapshot backup, user set rate limit from 128 to 32 but didn't see obvious improvements. Right now rate limiter only throttle the upload speed from TiVK to external storage. Need to investigate if rate limiter fails to control the snapshot/rocksdb read and see if we need to add another rate limiter on disk read.