pgsql: Fix logical replication slot initialization

Fix logical replication slot initialization

This was broken in commit 9c7d06d60680, which inadvertently gave the
wrong value to fast_forward in one StartupDecodingContext call. Fix by
flipping the value. Add a test for the obvious error, namely trying to
initialize a replication slot with an nonexistent output plugin.

While at it, move the CreateDecodingContext call earlier, so that any
errors are reported before sending the CopyBoth message.

Author: Dave Cramer <davecramer(at)gmail(dot)com>
Reviewed-by: Andres Freund <andres(at)anarazel(dot)de>
Discussion: https://postgr.es/m/CADK3HHLVkeRe1v4P02-5hj55H3_yJg3AEtpXyEY5T3wuzO2jSg@mail.gmail.com

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7326a7d63801673d3569def65cb7955d6486f254

Modified Files
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contrib/test_decoding/expected/slot.out | 2 ++
contrib/test_decoding/sql/slot.sql | 2 ++
src/backend/replication/logical/logical.c | 2 +-
src/backend/replication/walsender.c | 23 +++++++++++++----------
4 files changed, 18 insertions(+), 11 deletions(-)