pg_stat_statements: Add wal_buffers_full

pg_stat_statements: Add wal_buffers_full

wal_buffers_full tracks the number of times WAL buffers become full,
giving hints to be able to tune the GUC wal_buffers.

Up to now, this information was only available in pg_stat_wal.  With
this field available in WalUsage since eaf502747bac, exposing it in
pg_stat_statements is straight-forward, and it offers more granularity
at query level.

pg_stat_statements does not need a version bump as one has been done in
commit cf54a2c00254 for this development cycle.

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