Re-add GUC track_wal_io_timing

This commit is a rework of 2421e9a51d, about which Andres Freund has
raised some concerns as it is valuable to have both track_io_timing and
track_wal_io_timing in some cases, as the WAL write and fsync paths can
be a major bottleneck for some workloads.  Hence, it can be relevant to
not calculate the WAL timings in environments where pg_test_timing
performs poorly while capturing some IO data under track_io_timing for
the non-WAL IO paths.  The opposite can be also true: it should be
possible to disable the non-WAL timings and enable the WAL timings (the
previous GUC setups allowed this possibility).

track_wal_io_timing is added back in this commit, controlling if WAL
timings should be calculated in pg_stat_io for the read, fsync and write
paths, as done previously with pg_stat_wal.  pg_stat_wal previously
tracked only the sync and write parts (now removed), read stats is new
data tracked in pg_stat_io, all three are aggregated if
track_wal_io_timing is enabled.  The read part matters during recovery
or if a XLogReader is used.

Extra note: more control over if the types of timings calculated in
pg_stat_io could be done with a GUC that lists pairs of (IOObject,IOOp).

Reported-by: Andres Freund <andres@anarazel.de>
Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Co-authored-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/3opf2wh2oljco6ldyqf7ukabw3jijnnhno6fjb4mlu6civ5h24@fcwmhsgmlmzu

src/backend/utils/misc/postgresql.conf.sample

@@ -641,6 +641,7 @@
 #track_counts = on
 #track_cost_delay_timing = off
 #track_io_timing = off
+#track_wal_io_timing = off
 #track_functions = none			# none, pl, all
 #stats_fetch_consistency = cache	# cache, none, snapshot