Inconsistency in determining the timestamp of the db statfile.

We use the timestamp of the global statfile if we are not able to
determine it for a particular database either because the entry for
that database doesn't exist or there is an error while reading the
specific database entry. This was not taken care of while reading
other entries like ArchiverStats or SLRUStats. See
pgstat_read_db_statsfile_timestamp. I have observed this while
reviewing Sawada-San's patch related to logical replication stats [1].

I think this can only happen if due to some reason the statfile got
corrupt or we
have some bug in stats writing code, the chances of both are rare and even
if that happens we will use stale statistics.

The attached patch by Sawada-San will fix this issue. As the chances of this
the problem is rare and nobody has reported any issue related to this,
so it might be okay not to backpatch this.

Thoughts?

[1] - https://www.postgresql.org/message-id/CAA4eK1JBqQh9cBKjO-nKOOE%3D7f6ONDCZp0TJZfn4VsQqRZ%2BuYA%40mail.gmail.com

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With Regards,
Amit Kapila.