rgw: Resolve sync stuck in previous gen

rgw: Resolve sync stuck in previous gen

In case if any data log entries are missing for the older
generations, the sync server may not mark those shards as done
and can get stuck in that old gen for ever.

To avoid that, whenever a future gen entry is read, write the undone (shard,gen)
entry to error repo so that it can be processed and marked as done
and hence sync can progress eventually.

Signed-off-by: Soumya Koduri <skoduri@redhat.com>