rgw: implement ConfirmRemoveSelfBucketAccess header for bucket policy
According to https://docs.aws.amazon.com/AmazonS3/latest/API/API_PutBucketPolicy.html
root user should always have access to do Put/Get/DeleteBucketPolicy.
By implementing the `x-amz-confirm-remove-self-bucket-access` header
this privilege can also be dropped from the root user.