Add decoding of sequences to test_decoding

Add decoding of sequences to test_decoding

Commit 0da92dc530 improved the logical decoding infrastructure to handle
sequences, and did various changes to related parts (WAL logging etc.).
But it did not include any implementation of the new callbacks added to
OutputPluginCallbacks.

This extends test_decoding with two callbacks to decode sequences. The
decoding of sequences may be disabled using 'include-sequences', a new
option of the output plugin.

Author: Tomas Vondra, Cary Huang
Reviewed-by: Peter Eisentraut, Hannu Krosing, Andres Freund
Discussion: https://postgr.es/m/d045f3c2-6cfb-06d3-5540-e63c320df8bc@enterprisedb.com
Discussion: https://postgr.es/m/1710ed7e13b.cd7177461430746.3372264562543607781@highgo.ca