pgsql: Persist slot invalidation correctly

Persist slot invalidation correctly

We failed to save slot to disk after invalidating it, so the state was
lost in case of server restart or crash. Fix by marking it dirty and
flushing.

Also, if the slot is known invalidated we don't need to reason about the
LSN at all -- it's known invalidated. Only test the LSN if the slot is
known not invalidated.

Author: Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com>
Author: Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>
Reviewed-by: Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Discussion: https://postgr.es/m/17a69cfe-f1c1-a416-ee25-ae15427c69eb@oss.nttdata.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4ae08cd5fd19d566538005c15e7bf992ebae4c72

Modified Files
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src/backend/replication/slot.c | 9 +++++----
src/backend/replication/slotfuncs.c | 16 ++++++++--------
2 files changed, 13 insertions(+), 12 deletions(-)