pgsql: Remove read/sync fields from pg_stat_wal and GUC track_wal_io_ti

Remove read/sync fields from pg_stat_wal and GUC track_wal_io_timing

The four following attributes are removed from pg_stat_wal:
* wal_write
* wal_sync
* wal_write_time
* wal_sync_time

a051e71e28a1 has added an equivalent of this information in pg_stat_io
with more granularity as this now spreads across the backend types, IO
context and IO objects. So, keeping the same information in pg_stat_wal
has little benefits.

Another benefit of this commit is the removal of PendingWalStats,
simplifying an upcoming patch to add per-backend WAL statistics, which
already support IO statistics and which have access to the write/sync
stats data of WAL.

The GUC track_wal_io_timing, that was used to enable or disable the
aggregation of the write and sync timings for WAL, is also removed.
pgstat_prepare_io_time() is simplified.

Bump catalog version.
Bump PGSTAT_FILE_FORMAT_ID, due to the update of PgStat_WalStats.

Author: Bertrand Drouvot <bertranddrouvot(dot)pg(at)gmail(dot)com>
Discussion: https://postgr.es/m/Z7RkQ0EfYaqqjgz/@ip-10-97-1-34.eu-west-3.compute.internal

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2421e9a51d20bb83154e54a16ce628f9249fa907

Modified Files
--------------
doc/src/sgml/config.sgml | 22 ----------
doc/src/sgml/monitoring.sgml | 62 ---------------------------
src/backend/access/transam/xlog.c | 42 +++---------------
src/backend/access/transam/xlogreader.c | 2 +-
src/backend/access/transam/xlogrecovery.c | 2 +-
src/backend/catalog/system_views.sql | 4 --
src/backend/storage/buffer/bufmgr.c | 10 ++---
src/backend/storage/buffer/localbuf.c | 4 +-
src/backend/storage/smgr/md.c | 4 +-
src/backend/utils/activity/pgstat_io.c | 7 ++-
src/backend/utils/activity/pgstat_wal.c | 24 +----------
src/backend/utils/adt/pgstatfuncs.c | 20 ++-------
src/backend/utils/misc/guc_tables.c | 9 ----
src/backend/utils/misc/postgresql.conf.sample | 1 -
src/include/access/xlog.h | 1 -
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 6 +--
src/include/pgstat.h | 32 +-------------
src/test/regress/expected/rules.out | 6 +--
src/tools/pgindent/typedefs.list | 1 -
20 files changed, 32 insertions(+), 229 deletions(-)