Move wal_buffers_full from PgStat_PendingWalStats to WalUsage

Move wal_buffers_full from PgStat_PendingWalStats to WalUsage

wal_buffers_full has been introduced in pg_stat_wal in 8d9a935965f, as
some information providing metrics for the tuning of the GUC
wal_buffers.  WalUsage has been introduced before that in df3b181499.

Moving this field is proving to be beneficial for several reasons:
- This information can now be made available in more layers, providing
more granularity than just pg_stat_wal, on a per-query basis: EXPLAIN,
pgss and VACUUM/ANALYZE logs.
- A patch is under discussion to provide statistics for WAL at backend
level, and this move simplifies a bit the handling of pending
statistics.  The remaining data in PgStat_PendingWalStats now relates to
write/sync counters and times, with equivalents present in pg_stat_io,
that backend statistics are able to already track.  So this should cut
all the dependencies between PgStat_PendingWalStats and WAL stats at
backend level.

As of this change, wal_buffers_full only shows in pg_stat_wal.

Author: Bertrand Drouvot
Reviewed-by: Ilia Evdokimov
Discussion: https://postgr.es/m/Z6SOha5YFFgvpwQY@ip-10-97-1-34.eu-west-3.compute.internal