This fixes an issue where a db gets restored from bottomless and doesn't
get any writes until shutdown. At this point, the current generation is
the same as the restored one but the graceful shutdown process expects
to wait for that generation to be uploaded which never happens because
there are no writes. This change adds a snapshot generation emit call at
restore time to allow graceful shutdown to happen when there are no
writes without having to checkpoint and upload a new snapshot.
