rgw: use new BucketShard structure for index manipulation calls

Instead of recalculating the hash every call, do it once, and pass this
structure around. Also, will be used for logging changes into the data
log.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw: data changes log, log info by bucket shard id

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