pgsql: 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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/6b466bf5f2bea0c89fab54eef696bcfc7ecdafd7

Modified Files
--------------
.../expected/pg_stat_statements.out | 39 +++++++++++++++
.../pg_stat_statements--1.7--1.8.sql | 5 +-
contrib/pg_stat_statements/pg_stat_statements.c | 55 ++++++++++++++++++++--
.../pg_stat_statements/sql/pg_stat_statements.sql | 23 +++++++++
doc/src/sgml/pgstatstatements.sgml | 27 +++++++++++
5 files changed, 145 insertions(+), 4 deletions(-)