Change the default value of max-io-mb to 0 (unlimited)

[enjoy-binbin]

For now, the max-io-mb is 500MiB which may be too low for fast devices
like NVMe SSD, so it'd be better to set the io bandwidth to unlimited by default.

This closes #1711.

don't squash me (don't squash the below message)

i did a local test:

./build/kvrocks
redis-cli -p 6666 config get max-io-mb
1) "max-io-mb"
2) "0"

./build/kvrocks -c kvrocks.conf
redis-cli -p 6666 config get max-io-mb
1) "max-io-mb"
2) "0"

and note that we actually have a another default value:

const int64_t kIORateLimitMaxMb = 1024000;

void Storage::SetIORateLimit(int64_t max_io_mb) {
  if (max_io_mb == 0) {
    max_io_mb = kIORateLimitMaxMb;
  }
  rate_limiter_->SetBytesPerSecond(max_io_mb * static_cast<int64_t>(MiB));
}

[torwig]

LGTM

[git-hulk]

Thank you!