`sal::Bucket` no longer needs to wrap `RGWBucketEnt` to support user
bucket listings, so can be represented by `RGWBucketInfo` alone. the
bucket stats interfaces that relied on RGWBucketEnt internally now
return their result as either `RGWBucketEnt` or `RGWStorageStats`
Signed-off-by: Casey Bodley <cbodley@redhat.com>