cls_rgw: clean up CLSRGWConcurrentIO

Class is no longer a template, and keeps a map of oids by shard_id. Call
issue_op() using both shard_id and oids. Shard id is used for mapping
the results in the derived classes.

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

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