rgw: fix a few regressions

 - need to increase olh epoch if converting plain entry, otherwise
   ordering is skewed
 - if entry is olh (e.g., colocated data + olh), need to make sure we
   write olh idtag xattr
 - completely revalidate obj state after writing it, otherwise attrs
   won't have idtag, and later check to verify if it's olh will fail

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