Stop events even with stale connected participants

`ServerSession.stopWithDelay` was using `getNumConnectedParticipants` to
check whether new participants had joined after all sockets had
disconnected from a session.  However, non-socketed participants would
appear as connectedParticipants even after they'd left, if no socketed
participant had yet informed the session of their departure.

Instead, use `hangout-stop-request-time` as a sentinel indicating that
the session is still to be stopped.  That value is cleared whenever a
new participant joins, so is a more reliable indicator that the session
is still slated for stoppage.
Minor fixes to two tests