Add information about WAL buffers full to VACUUM/ANALYZE (VERBOSE)

Add information about WAL buffers full to VACUUM/ANALYZE (VERBOSE)

This commit adds the information about the number of times WAL buffers
have been full to the logs generated by VACUUM/ANALYZE (VERBOSE) and in
the logs generated by autovacuum, complementing the existing information
stored by WalUsage.

This is the last part of the backend code where the value of
wal_buffers_full can be reported, similarly to all the other fields of
WalUsage.  320545bfcfee and ce5bcc4a9f26 have done the same for EXPLAIN
and pgss.

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