Add data for WAL in pg_stat_io and backend statistics

Add data for WAL in pg_stat_io and backend statistics

This commit adds WAL IO stats to both pg_stat_io view and per-backend IO
statistics (pg_stat_get_backend_io()).  This change is possible since
f92c854cf406, as WAL IO is not counted in blocks in some code paths
where its stats data is measured (like WAL read in xlogreader.c).

IOContext gains IOCONTEXT_INIT and IOObject IOOBJECT_WAL, with the
following combinations allowed:
- IOOBJECT_WAL/IOCONTEXT_NORMAL is used to track I/O operations done on
already-created WAL segments.
- IOOBJECT_WAL/IOCONTEXT_INIT is used for tracking I/O operations done
when initializing WAL segments.

The core changes are done in pg_stat_io.c, backend statistics inherit
them.  Backend statistics and pg_stat_io are now available for the WAL
writer, the WAL receiver and the WAL summarizer processes.

I/O timing data is controlled by the GUC track_io_timing, like the
existing data of pg_stat_io for consistency.  The timings related to
IOOBJECT_WAL show up if the GUC is enabled (disabled by default).

Bump pgstats file version, due to the additions in IOObject and
IOContext, impacting the amount of data written for the fixed-numbered
IO stats kind in the pgstats file.

Author: Nazir Bilal Yavuz
Reviewed-by: Bertrand Drouvot, Nitin Jadhav, Amit Kapila, Michael
Paquier, Melanie Plageman, Bharath Rupireddy
Discussion: https://postgr.es/m/CAN55FZ3AiQ+ZMxUuXnBpd0Rrh1YhwJ5FudkHg=JU0P+-W8T4Vg@mail.gmail.com