1. fix concurrency bug of s3 fs benchmark tool, to avoid crash on multi thread. 2. Add `prefetch_read` operation to test prefetch reader. 3. add `AWS_EC2_METADATA_DISABLED` env in `start_be.sh` to avoid call ec2 metadata when creating s3 client. 4. add `AWS_MAX_ATTEMPTS` env in `start_be.sh` to avoid warning log of s3 sdk.