pgsql: Avoid special XID snapshotConflictHorizon values.

Avoid special XID snapshotConflictHorizon values.

Don't allow VACUUM to WAL-log the value FrozenTransactionId as the
snapshotConflictHorizon of freezing or visibility map related WAL
records.

The only special XID value that's an allowable snapshotConflictHorizon
is InvalidTransactionId, which is interpreted as "record definitely
doesn't require a recovery conflict".

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WznuNGSzF8v6OsgjaC5aYsb3cZ6HW6MLm30X0d65cmSH6A@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6daeeb1f9196e5d59aa7d5c9ef7ed67b3f3ac4b5

Modified Files
--------------
src/backend/access/heap/vacuumlazy.c | 8 +++++---
src/backend/storage/ipc/standby.c    | 1 +
2 files changed, 6 insertions(+), 3 deletions(-)