New GUC, track_iotiming, to track I/O timings.

Currently, the only way to see the numbers this gathers is via
EXPLAIN (ANALYZE, BUFFERS), but the plan is to add visibility through
the stats collector and pg_stat_statements in subsequent patches.

Ants Aasma, reviewed by Greg Smith, with some further changes by me.

src/backend/utils/misc/guc.c

@@ -1018,6 +1018,15 @@ static struct config_bool ConfigureNamesBool[] =
 		true,
 		NULL, NULL, NULL
 	},
+	{
+		{"track_iotiming", PGC_SUSET, STATS_COLLECTOR,
+			gettext_noop("Collects timing information for database IO activity."),
+			NULL
+		},
+		&track_iotiming,
+		false,
+		NULL, NULL, NULL
+	},
 
 	{
 		{"update_process_title", PGC_SUSET, STATS_COLLECTOR,