Add STREAM_START/STREAM_STOP for transactional messages during decoding.

Add STREAM_START/STREAM_STOP for transactional messages during decoding.

In test_decoding module, when skip_empty_xacts option was specified, add
stream_start/stop for streaming transactional messages. This makes the
handling of transactional messages stream consistent irrespective of
whether skip_empty_xacts option was specified.

Commit 26dd0284b9 made a similar change for non-streaming messages but
forgot to update the streaming cases.

Author: Peter Smith
Reviewed-by: Amit Kapila
Discussion: http://postgr.es/m/OS0PR01MB5716AEBD2988F8F5E9D5985794DFA@OS0PR01MB5716.jpnprd01.prod.outlook.com