rgw, cls_rgw: keep shard ids with oids

Instead of just having the list of oids, keep the shard ids together, so
that we can know on which shard the operation happened.
Bucket markers are just using the shard numeric id, instead of the
bucket instance shard id. This makes it easier to parse the markers
appropriately.

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