pgsql: Replace replication slot's invalidated_at LSN with an enum

Replace replication slot's invalidated_at LSN with an enum

This is mainly useful because the upcoming logical-decoding-on-standby feature
adds further reasons for invalidating slots, and we don't want to end up with
multiple invalidated_* fields, or check different attributes.

Eventually we should consider not resetting restart_lsn when invalidating a
slot due to max_slot_wal_keep_size. But that's a user visible change, so left
for later.

Increases SLOT_VERSION, due to the changed field (with a different alignment,
no less).

Reviewed-by: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/20230407075009.igg7be27ha2htkbt@awork3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/15f8203a5975d6b9b78e2c64e213ed964b50c044

Modified Files
--------------
src/backend/replication/slot.c      | 28 ++++++++++++++++++++++++----
src/backend/replication/slotfuncs.c |  8 +++-----
src/include/replication/slot.h      | 15 +++++++++++++--
src/tools/pgindent/typedefs.list    |  1 +
4 files changed, 41 insertions(+), 11 deletions(-)