pgsql: Fix skip-empty-xacts with sequences in test_decoding

Fix skip-empty-xacts with sequences in test_decoding

Regression tests need to use skip-empty-xacts = false, because there
might be accidental concurrent activity (like autovacuum), particularly
on slow machines. The tests added by 80901b3291 failed to do that in a
couple places, triggering occasional failures on buildfarm.

Fixing the tests however uncovered a bug in the code, because sequence
callbacks did not handle skip-empty-xacts properly. For trasactional
increments we need to check/update the xact_wrote_changes flag, and emit
the BEGIN if it's the first change in the transaction.

Reported-by: Andres Freund
Discussion: https://postgr.es/m/20220212220413.b25amklo7t4xb7ni%40alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b779d7d8fdae088d70da5ed9fcd8205035676df3

Modified Files
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contrib/test_decoding/expected/sequence.out | 12 +++++-------
contrib/test_decoding/sql/sequence.sql      |  8 ++++----
contrib/test_decoding/test_decoding.c       | 22 ++++++++++++++++++++++
3 files changed, 31 insertions(+), 11 deletions(-)