pgsql: 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

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/0188bb82531f1b0ae3648fb81a4bd4a4f6242127

Modified Files
--------------
src/backend/replication/slot.c            |  1 +
src/backend/replication/slotfuncs.c       | 11 ++++++++++-
src/include/access/xlog.h                 |  2 +-
src/include/replication/slot.h            |  3 +++
src/test/recovery/t/019_replslot_limit.pl |  2 +-
5 files changed, 16 insertions(+), 3 deletions(-)