9278: Set expires_at=now if a client sets it to a time in the past.

9278: Set expires_at=now if a client sets it to a time in the past.

The definition of "now" in the default collection scope changes from
current_timestamp (time the current transaction started) to
statement_timestamp() (time the current statement started) so a test
case can expire a collection and then confirm that it is not in the
default scope, all within a single test transaction.