Save slot's restart_lsn when invalidated due to size

Save slot's restart_lsn when invalidated due to size

We put it aside as invalidated_at, which let us show "lost" in
pg_replication slot.  Prior to this change, the state value was reported
as NULL.

Backpatch to 13.

Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/20200617.101707.1735599255100002667.horikyota.ntt@gmail.com
Discussion: https://postgr.es/m/20200407.120905.1507671100168805403.horikyota.ntt@gmail.com