Add pg_stat_wal statistics view.

This view shows the statistics about WAL activity. Currently it has only
two columns: wal_buffers_full and stats_reset. wal_buffers_full column
indicates the number of times WAL data was written to the disk because
WAL buffers got full. This information is useful when tuning wal_buffers.
stats_reset column indicates the time at which these statistics were
last reset.

pg_stat_wal view is also the basic infrastructure to expose other
various statistics about WAL activity later.

Bump PGSTAT_FILE_FORMAT_ID due to the change in pgstat format.

Bump catalog version.

Author: Masahiro Ikeda
Reviewed-by: Takayuki Tsunakawa, Kyotaro Horiguchi, Amit Kapila, Fujii Masao
Discussion: https://postgr.es/m/188bd3f2d2233cf97753b5ced02bb050@oss.nttdata.com

src/backend/postmaster/checkpointer.c

@@ -504,6 +504,9 @@ CheckpointerMain(void)
 		 */
 		pgstat_send_bgwriter();
 
+		/* Send WAL statistics to the stats collector. */
+		pgstat_send_wal();
+
 		/*
 		 * If any checkpoint flags have been set, redo the loop to handle the
 		 * checkpoint without sleeping.