cls/rgw: adjust types and ops

cls/rgw: adjust types and ops

 - don't keep data bufferlist in struct cls_rgw_bucket_instance_entry
 - remove rgw_get_bucket_instance

We need to keep the reshard info as part of the bucket index header (for
each source bucket shard), and not in a separate object. Otherwise it'll
be impossible to have the resharding check atomic as part of the regular
bucket index updates.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>