Ceph

Normally the missing set is reconstructed on OSD startup by scanning the log and adding objects which are missing or at the wrong version. However, it is possible to have a missing set item which is not in the log due to a divergent log entry since we add a missing set element at prior_version which might be prior to log_tail. We need to persist a record of such cases when we persist the log removing the divergent entries. Perhaps a separate list alongside the log?