Don't allow GetTransactionSnapshot() in logical decoding
A historic snapshot should only be used for catalog access, not
general queries. We never call GetTransactionSnapshot() during logical
decoding, which is good because it wouldn't be very sensible, so the
code to deal with that was unreachable and untested. Turn it into an
error, to avoid doing that in the future either.
Discussion: https://www.postgresql.org/message-id/
a868fe78-ddb4-4b0a-9b96-
873d91d93cfd@iki.fi