Show WAL write and fsync stats in pg_stat_io

Hi,

This is a WIP patch to add WAL write and fsync stats to pg_stat_io
view. There is a track_io_timing variable to control pg_stat_io
timings and a track_wal_io_timing variable to control WAL timings. I
couldn't decide on which logic to enable WAL timings on pg_stat_io.
For now, both pg_stat_io and track_wal_io_timing are needed to be
enabled to track WAL timings in pg_stat_io.

Also, if you compare WAL stats in pg_stat_wal and pg_stat_io; you can
come across differences. These differences are caused by the
background writer's WAL stats not being flushed. Because of that,
background writer's WAL stats are not seen in pg_stat_wal but in
pg_stat_io. I already sent a patch [1] to fix that.

[1] https://www.postgresql.org/message-id/CAN55FZ2FPYngovZstr%3D3w1KSEHe6toiZwrurbhspfkXe5UDocg%40mail.gmail.com

Any kind of feedback would be appreciated.

Regards,
Nazir Bilal Yavuz
Microsoft