Add statistics refresh materialized view

Hi.

This is a proposal for a new feature in statistics collector.
I think we need to add statistics about refresh matview to
pg_stat_all_tables view.

When the "REFRESH MATERIALIZED VIEW" was executed, the number of times
it was executed
and date it took were not recorded anywhere.

"pg_stat_statements" can be used to get the number of executions and the
date and time of execution,
but this information is statement-based, not view-based.
Also, that method requires the high cost of "pg_stat_statements".

This patch will add statistics(count, last time) about "REFRESH
MATERIALIZED VIEW"
to pg_stat_all_tables(pg_stat_user_tables, [pg_stat_sys_tables]).

What do you think?

Regards,
Seino Yuki