not all pg_stat_database fields reset after pg_stat_reset()

After calling pg_stat_reset, some of the database stats fields are not
actually reset, the value is preserved. I've found the bug is actually in
pgstat_recv_resetcounter function, where only some of the
PgStat_StatDBEntry fields are reset to 0.

This is true for those 6 fields:

n_tuples_returned
n_tuples_fetched
n_tuples_inserted
n_tuples_updated
n_tuples_deleted
last_autovac_time

The docs for "pg_stat_reset()" say "Reset all statistics counters for the
current database to zero (requiret superuser privileges)" and I don't see
any reason why these fields should not be reset, so I guess it's a bug.

See the patch attached (against current dev version).

regards
Tomas