Use Walk(), which is a recursive listing with versioning, to check if
the bucket has some objects before being removed. This is beneficial
because the bucket can contain multiple dangling objects in multiple
drives.
Also, this will prevent a bug where a bucket is deleted in a deployment
that has many erasure sets but the bucket contains one or few objects
not spread to enough erasure sets.
Currently, bucket metadata is being loaded serially inside ListBuckets
Objet API. Fix that by loading the bucket metadata as the number of
erasure sets * 10, which is a good approximation.