lib: Set thread subject at the same time as setting thread->{oldest,newest}

lib: Set thread subject at the same time as setting thread->{oldest,newest}

We really want to change the thread subject at the same time we set
the date, (if the sort order indicates this is necessary). The
previous code for setting the thread subject was sensitive on the
query sort when adding matching messages. An independent bug fix is
about to change that query sort order, so we remove the dependency on
it here.