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