rgw: bucket index log fixes

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: share object tag and index tag

object tag is now being written to the index, so that both
object and index hold the same tag. This is needed so that we
could know whether object and index refer to the same instance.

Also cleanup old legacy code that did atomic ops through cloning
objects.

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