Allow pg_stat_statements to track WAL usage statistics.

Allow pg_stat_statements to track WAL usage statistics.

This commit adds three new columns in pg_stat_statements output to
display WAL usage statistics added by commit df3b181499.

This commit doesn't bump the version of pg_stat_statements as the
same is done for this release in commit 17e0328224.

Author: Kirill Bychik and Julien Rouhaud
Reviewed-by: Julien Rouhaud, Fujii Masao, Dilip Kumar and Amit Kapila
Discussion: https://postgr.es/m/CAB-hujrP8ZfUkvL5OYETipQwA=e3n7oqHFU=4ZLxWS_Cza3kQQ@mail.gmail.com