pgsql: Add BEGIN/COMMIT for transactional messages during decoding.

Add BEGIN/COMMIT for transactional messages during decoding.

In test_decoding module, when skip_empty_xacts option was specified, add
BEGIN/COMMIT for transactional messages. This makes the handling of
transactional messages consistent irrespective of whether skip_empty_xacts
option was specified.

We decided not to backpatch this change because skip_empty_xacts is
primarily used to have consistent test results across different runs and
this change won't help with that.

Author: Vignesh C
Reviewed-by: Ashutosh Bapat, Hou Zhijie
Discussion: https://postgr.es/m/CAExHW5ujRhbOz6_aTq_jQA8NjeFqq9d_8G9viShWvXx8gdSXiQ@mail.gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/26dd0284b98f6bf730dc1f7f7e1f917525d71eda

Modified Files
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contrib/test_decoding/expected/messages.out | 10 ++++++++--
contrib/test_decoding/sql/messages.sql      |  2 +-
contrib/test_decoding/test_decoding.c       | 12 ++++++++++++
3 files changed, 21 insertions(+), 3 deletions(-)