Adding new PgStat_WalCounters structure in pgstat.h

Adding new PgStat_WalCounters structure in pgstat.h

This new structure contains the counters and the data related to the WAL
activity statistics gathered from WalUsage, separated into its own
structure so as it can be shared across more than one Stats structure in
pg_stat.h.

This refactoring will be used by an upcoming patch introducing
backend-level WAL statistics.

Bump PGSTAT_FILE_FORMAT_ID.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/Z3zqc4o09dM/Ezyz@ip-10-97-1-34.eu-west-3.compute.internal